Time-dependent density functional theory: past, present, and future

An o-Phenylene Bridged Noninnocent Bis-Azopyridyl Ligand and Its Copper Complexes: Synthesis, Characterization of Electron Transfer Events, and Use of the Cu Complexes for Oxidation of Alcohols

In this report, an o-phenylene-bridged tetradented redox-noninnocent bis-azopyridyl ligand [L] and its copper complexes [1] and [2] were synthesized and characterized. The electron transfer events of [L], as well as [1] and [2], were characterized by single-crystal X-ray structure determination, various spectroscopic studies, and DFT calculations. While [1] [[L]Cu(II)Cl2] has unreduced [L], [2], [[L]•-Cu(I)Cl] contains a one-electron-reduced ligand [L]•- , which is antiferromagnetically coupled with the unpaired spin on Cu(II). Reduction of [2] by one electron generated the complex [2]•-, which remained in an electronically bistable situation in the form of valence tautomers: [2A]•-, [[L]•-Cu(I)Cl]•- and [2B]•-, [[L]2-Cu(II)Cl]•-. Further one-electron reduction of [2]•- generated a mixture of Cu complexes: [2A]-, [[L]2-Cu(I)]- and [2B]-, [[L]3•-Cu(II)]-. Complexes [1] and [2] were examined for the catalytic oxidation of alcohols. The complex [2] was more efficient than [1]. The protocol was highly efficient and versatile with both primary as well as secondary aromatic and aliphatic alcohols. Mechanistic investigations showed that the complex [2] generated [[L]•-Cu(I)OCH2Ph]•- (A) as the active catalyst, which subsequently, through its ligand-based redox events, acted as the catalyst over the course of the reaction.

Improving Aufbau Suppressed Coupled Cluster through Perturbative Analysis

Guided by perturbative analysis, we improve the accuracy of Aufbau suppressed coupled cluster theory in simple single excitations, multiconfigurational single excitations, and charge transfer excitations while keeping the cost of its leading-order terms precisely in line with ground-state coupled cluster. Combining these accuracy improvements with a more efficient implementation based on spin adaptation, we observe high accuracy in a large test set of single excitations and, in particular, a mean unsigned error for charge transfer states that outperforms equation-of-motion coupled cluster theory by 0.25 eV. We discuss how these results are achieved via a systematic identification of which amplitudes to prioritize for single- and multiconfigurational excited states, and how this prioritization differs in important ways from the ground-state theory. In particular, our data show that a partial linearization of the theory increases accuracy by mitigating unwanted side effects of Aufbau suppression.

Excited-state-specific Kohn-Sham formalism for the asymmetric Hubbard dimer

Building on our recent study [Giarrusso and Loos, J. Phys. Chem. Lett. 14, 8780 (2023)], we explore the generalization of the ground-state Kohn-Sham (KS) formalism of density-functional theory (DFT) to the (singlet) excited states of the asymmetric Hubbard dimer at half-filling. While we found that the KS-DFT framework can be straightforwardly generalized to the highest-lying doubly excited state, the treatment of the first excited state presents significant challenges. In particular, using a density-fixed adiabatic connection, we show that the density of the first excited state lacks non-interacting v-representability. However, by employing an analytic continuation of the adiabatic path, we demonstrate that the density of the first excited state can be generated by a complex-valued external potential in the non-interacting case. More practically, by performing state-specific KS calculations with exact and approximate correlation functionals-each state possessing a distinct correlation functional-we observe that spurious stationary solutions of the KS equations may arise due to the approximate nature of the functional.

Assessment of Charge Transfer Energies of Noncovalently Bounded Ar-TCNE Complexes Using Range-Separated Density Functionals and Double-hybrid Density Functionals

Charge Transfer (CT) molecular complexes have recently received much attention in a broad variety of fields. The time-dependent density functional theory (TDDFT), which is essential for studying CT complexes, is a well-established tool to study the excited states of relatively large molecular systems. However, when dealing with donor-acceptor molecules with CT characteristics, TDDFT calculations based on standard functionals can severely underestimate the excitation energies. The TDDFT methodology, combined with range-separated DFT and range-separated double-hybrid DFT functionals, had previously been used by different research groups to reliably predict the excitation energies of different charge transfer molecular complexes. We follow the same path to calculate the excited state charge transfer energy of some selected molecular complexes, such as, Ar-TCNE (TCNE=tetracyanoethylene; Ar= benzene, naphthalene, anthracene, etc.). The interactions between the donor-acceptor moieties of these molecular complexes are also studied and the relationship between the interaction and the charge transfer energies are shown here.

Review of the foundations of time-dependent density-functional theory (TDDFT)

Time-dependent density-functional theory (TDDFT) is deemed to be a formally rigorous way of dealing with the time-evolution of a many-electron system at the level of electron densities rather than the underlying wavefunctions, which in turn provides an, in principle, exact density-based approach to the treatment of electron excitations in atoms and molecules. This claim has not remained unchallenged, and a detailed account of the relevant criticism is given in this paper. In view of our analysis one has to face the conclusion that there is currently no valid foundation for TDDFT, and expectations of finding a remedy here are hardly justified.

Revealing the intrinsic mechanism of the influence of different rings and oxidized structures on the room temperature phosphorescence

In this work, we used density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to study the mechanism of pure organic room temperature phosphorescence emission. The effects on the electronic structure and photochemical properties of thiophene and diketone derivatives with different cyclic and oxidized structures. The result suggests that varying ring configurations and oxidation products significantly influence the photochemical characteristics of thiophene and diketone derivatives. The complex experiences conformational distortion along with the oxidation product, which causes notable alterations in the energy gap and charge density of its frontier molecular orbitals. An oxidation process significantly distorts the molecular structure of the compound, leading to excited singlet and excited triplet states structural similarities. With energy gaps dropping from 0.22 eV to 0.05 eV and spin-orbit coupling constants rising from 0.42 cm-1 to 57.48 cm-1, the excited singlet and excited triplet states share structures and charge distributions that increase the energy level channels appropriate for intersystem crossing. Therefore, this work can provide theoretical support for the design and structural optimization of highly efficient pure organic phosphorescent materials.

Multiconfigurational short-range on-top pair-density functional theory

We present the theory and implementation of a fully variational wave function-density functional theory (DFT) hybrid model, which is applicable to many cases of strong correlation. We denote this model as the multiconfigurational self-consistent on-top pair-density functional theory (MC-srPDFT) model. We have previously shown how the multiconfigurational short-range DFT (MC-srDFT) hybrid model can describe many multiconfigurational cases of any spin symmetry and also state-specific calculations on excited states [Hedegård et al., J. Chem. Phys. 148(21), 214103 (2018)]. However, the srDFT part of the MC-srDFT has some deficiencies that it shares with Kohn-Sham DFT; in particular, (1) self-interaction errors (albeit reduced because of the range separation), (2) that different MS states incorrectly become non-degenerate, and (3) that singlet and non-singlet states dissociating to the same open-shell fragments incorrectly lead to different electronic energies at dissociation. The model that we present in this paper corrects these deficiencies by introducing the on-top pair density as an auxiliary variable replacing the spin density. Unlike other models in the literature, our model is fully variational and employs a long-range version of the on-top pair density. The implementation is a second-order optimization algorithm ensuring robust convergence to both ground and excited states. We show how MC-srPDFT solves the mentioned challenges by sample calculations on the ground state singlet curve of H2, N2, and Cr2 and the lowest triplet curves for N2 and Cr2. Furthermore, the rotational barrier for ethene is investigated for the S0 and T1 states. The calculations show correct degeneracy between the singlet and triplet curves at dissociation and the results are invariant to the choice of the MS value for the triplet curves.

Redox Noninnocent Copper(I) Complex Where Metal Is a Spectator and Ligand Is an Actor in the Glaser Coupling Reaction of Alkynes

An extended trisazo dipyridyl ligand, L, and its copper(I) complex, [1]+, were synthesized and fully characterized. Complex [1]+ has five coordination geometry satisfied by ligand L. L has a low-lying π* orbital; thus, [1]+ showed very facile multiple ligand-based redox events. Moreover, due to the strong π-acceptor nature of L, the Cu(II)/Cu(I) redox potential of [1]+ was anodic (0.62 V). The redox events of both L and [1]+ were characterized using various spectroscopic studies and density functional theory (DFT) calculations. Taking advantage of the multiple facile ligand-based reductions in [1]+, the Glaser coupling reaction of terminal alkynes was explored. Various kinds of alkynes were found to be effective when using [1]+ as a precatalyst. The mechanism of the reaction was investigated thoroughly by several controlled experiments, isolation, and characterization of the intermediates using various spectroscopic studies as well as by single-crystal X-ray structure determination. These studies showed that the L in [1]+ acted not only in the electron transfer events but also as a locus for binding the substrate, breaking and forming bonds, and, finally, releasing the product. Thus, here, the metal mainly acted as a spectator and ligand L acted as an actor.

Deep Learning for Generating Phase-Conditioned Infrared Spectra

Infrared (IR) spectroscopy is an efficient method for identifying unknown chemical compounds. To accelerate IR spectrum analysis, various calculation and machine learning methods for simulating IR spectra of molecules have been studied in chemical science. However, existing calculation and machine learning methods assumed a rigid constraint that all molecules are in the gas phase, i.e., they overlooked the phase dependency of the IR spectra. In this paper, we propose an efficient phase-aware machine learning method to generate phase-conditioned IR spectra from 2D molecular structures. To this end, we devised a phase-aware graph neural network and combined it with a transformer decoder. To the best of our knowledge, the proposed method is the first IR spectrum generator that can generate the phase-conditioned IR spectra of real-world complex molecules. The proposed method outperformed state-of-the-art methods in the tasks of generating IR spectra on a benchmark dataset containing experimentally measured 11,546 IR spectra of 10,288 unique molecules. All implementations of the proposed method are publicly available at https://github.com/ngs00/PASGeN.

Simulating Real-Time Molecular Electron Dynamics Efficiently Using the Time-Dependent Density Matrix Renormalization Group

Compared to ground-state electronic structure optimizations, accurate simulations of molecular real-time electron dynamics are usually much more difficult to perform. To simulate electron dynamics, the time-dependent density matrix renormalization group (TDDMRG) has been shown to offer an attractive compromise between accuracy and cost. However, many simulation parameters significantly affect the quality and efficiency of a TDDMRG simulation. So far, it is unclear whether common wisdom from ground-state DMRG carries over to the TDDMRG, and a guideline on how to choose these parameters is missing. Here, in order to establish such a guideline, we investigate the convergence behavior of the main TDDMRG simulation parameters, such as time integrator, the choice of orbitals, and the choice of matrix-product-state representation for complex-valued nonsinglet states. In addition, we propose a method to select orbitals that are tailored to optimize the dynamics. Lastly, we showcase the TDDMRG by applying it to charge migration ionization dynamics in furfural, where we reveal a rapid conversion from an ionized state with a σ character to one with a π character within less than a femtosecond.

Benchmark computations of nearly degenerate singlet and triplet states of N-heterocyclic chromophores. I. Wavefunction-based methods

In this paper, we investigate the role of electron correlation in predicting the S1-S0 and T1-S0 excitation energies and, hence, the singlet-triplet gap (ΔEST) in a set of cyclazines, which act as templates for potential candidates for fifth generation organic light emitting diode materials. This issue has recently garnered much interest with the focus being on the inversion of the ΔEST, although experiments have indicated near degenerate levels with both positive and negative being within the experimental error bar [J. Am. Chem. Soc. 102, 6068 (1980), J. Am. Chem. Soc. 108, 17(1986)]. We have carried out a systematic and exhaustive study of various excited state electronic structure methodologies and identified the strengths and shortcomings of the various approaches and approximations in view of this challenging case. We have found that near degeneracy can be achieved either with a proper balance of static and dynamic correlation in multireference theories or with state-specific orbital corrections, including its coupling with correlation. The role of spin contamination is also discussed. Eventually, this paper seeks to produce benchmark numbers for establishing cost-effective theories, which can then be used for screening derivatives of these templates with desirable optical and structural properties. Additionally, we would like to point out that the use of domain-based local pair natural orbital-similarity transformed EOM-coupled cluster singles and doubles as the benchmark for ΔEST [as used in J. Phys. Chem. A 126(8), 1378 (2022), Chem. Phys. Lett. 779, 138827 (2021)] is not a suitable benchmark for these classes of molecules.

Pressure induced emission enhancement (PIEE) in solid-state 2,3,4,5-tetraphenylthiophene: a QM/MM study

Organic fluorophores exhibit pressure-dependent behaviors that are often irregular and contingent upon the specific system. Elucidating how pressure influences these behaviors is essential for the accurate design of piezochromic materials. Here, we conducted an exhaustive theoretical analysis of the excited-state decay processes of crystalline 2,3,4,5-tetraphenylthiophene (TPT) at high pressure through a combined quantum mechanics and molecular mechanics (QM/MM) method. The study revealed that the fluorescence quantum yield experiences a pronounced initial increase owing to the decrease of nonradiative decay IC rate (k ic). The suppression of low-frequency modes results in the decrease of λ e, and then reduces the electron-vibration couplings, and finally slows down the non-radiative process. Our research provides detailed mechanism analyses on PIEE properties of solid state TPT, aiding the rational design of advanced PIEE materials.

Electronic Vector Potential from the Exact Factorization of a Complex Wavefunction

We generalize the definitions of local scalar potentials namedυ kin ${\upsilon _{{\rm{kin}}} }$ andυ N - 1 ${\upsilon _{N - 1} }$ , which are relevant to properly describe phenomena such as molecular dissociation with density-functional theory, to the case in which the electronic wavefunction corresponds to a complex current-carrying state. In such a case, an extra term in the form of a vector potential appears which cannot be gauged away. Both scalar and vector potentials are introduced via the exact factorization formalism which allows us to express the given Schrödinger equation as two coupled equations, one for the marginal and one for the conditional amplitude. The electronic vector potential is directly related to the paramagnetic current density carried by the total wavefunction and to the diamagnetic current density in the equation for the marginal amplitude. An explicit example of this vector potential in a triplet state of two non-interacting electrons is showcased together with its associated circulation, giving rise to a non-vanishing geometric phase. Some connections with the exact factorization for the full molecular wavefunction beyond the Born-Oppenheimer approximation are also discussed.

RNA-Activatable Near-Infrared Photosensitizer for Cancer Therapy

Photodynamic therapy (PDT) has recently come to the forefront as an exceptionally powerful and promising method for the treatment of cancer. Existing photosensitizers are predominantly engineered to target diverse biomolecules, including proteins, DNA, lipids, and carbohydrates, and have proven to greatly enhance the efficacy or specificity of PDT. However, it is noteworthy that there exists a conspicuous scarcity of photosensitizers specifically designed to target RNAs. Recognizing the crucial and multifaceted roles played by RNAs in various cellular processes and disease states, we have ventured into the development of a novel RNA-targeting photosensitizer, named Se-718, designed specifically for PDT-based cancer therapy. Se-718 has been engineered to exhibit a high molar absorption coefficient in the NIR region, which is crucial for effective PDT. More importantly, Se-718 has demonstrated a distinct RNA-targeting capability, as evidenced through rigorous testing in both circular dichroism and fluorescence experiments. Furthermore, Se-718 has been shown to display both type I and type II photodynamic properties. This unique characteristic enables the efficient killing of cancer cells under a wide range of oxygen conditions, both normoxic (21% O2) and hypoxic (2% O2). The IC50 of Se-718 can be as low as 100 nM, and its light-to-dark toxicity ratio is an impressive 215 times higher, outperforming most photosensitizers currently available. Moreover, in vivo studies conducted with tumor-bearing mice have demonstrated the excellent antitumor effects and high safety profile of Se-718. Considering the outstanding PDT efficacy of Se-718, we are optimistic that the development of RNA-targeting photosensitizers may provide an innovative and highly effective option for cancer therapeutics in the near future.

Convergence Analysis of the Stochastic Resolution of Identity: Comparing Hutchinson to Hutch++ for the Second-Order Green's Function

Stochastic orbital techniques offer reduced computational scaling and memory requirements to describe ground and excited states at the cost of introducing controlled statistical errors. Such techniques often rely on two basic operations, stochastic trace estimation and stochastic resolution of identity, both of which lead to statistical errors that scale with the number of stochastic realizations (Nξ) as N ξ - 1 . Reducing the statistical errors without significantly increasing Nξ has been challenging and is central to the development of efficient and accurate stochastic algorithms. In this work, we build upon recent progress made to improve stochastic trace estimation based on the ubiquitous Hutchinson's algorithm and propose a two-step approach for the stochastic resolution of identity, in the spirit of the Hutch++ method. Our approach is based on employing a randomized low-rank approximation followed by a residual calculation, resulting in statistical errors that scale much better than N ξ - 1 . We implement the approach within the second-order Born approximation for the self-energy in the computation of neutral excitations and discuss three different low-rank approximations for the two-body Coulomb integrals. Tests on a series of hydrogen dimer chains with varying lengths demonstrate that the Hutch++-like approximations are computationally more efficient than both deterministic and purely stochastic (Hutchinson) approaches for low error thresholds and intermediate system sizes. Notably, for arbitrarily large systems, the Hutchinson-like approximation outperforms both deterministic and Hutch++-like methods.

Theoretical insights of the pharmaceutical compound fluoxetine in water: Role in direct photolysis and indirect photolysis by free radicals

The frequent detection of pharmaceutical compounds in the environment has led to a growing awareness, which may pose a major threat to the aquatic environment. In this study, photodegradation (direct and indirect photolysis) of two different dissociation states of fluoxetine (FLU) was investigated in water, mainly including the determination of photolytic transition states and products, and the mechanisms of indirect photodegradation with ·OH, CO3*- and NO3*. The main direct photolysis pathways are defluorination and C-C bond cleavage. In addition, the indirect photodegradation of FLU in water is mainly through the reactions with ·OH and NO3*, and the photodegradation reaction with CO3*- is relatively difficult to occur in the water environment. Our results provide a theoretical basis for understanding the phototransformation process of FLU in the water environment and assessing its potential risk.

Efficient exact exchange using Wannier functions and other related developments in planewave-pseudopotential implementation of RT-TDDFT

The plane-wave pseudopotential (PW-PP) formalism is widely used for the first-principles electronic structure calculation of extended periodic systems. The PW-PP approach has also been adapted for real-time time-dependent density functional theory (RT-TDDFT) to investigate time-dependent electronic dynamical phenomena. In this work, we detail recent advances in the PW-PP formalism for RT-TDDFT, particularly how maximally localized Wannier functions (MLWFs) are used to accelerate simulations using the exact exchange. We also discuss several related developments, including an anti-Hermitian correction for the time-dependent MLWFs (TD-MLWFs) when a time-dependent electric field is applied, the refinement procedure for TD-MLWFs, comparison of the velocity and length gauge approaches for applying an electric field, and elimination of long-range electrostatic interaction, as well as usage of a complex absorbing potential for modeling isolated systems when using the PW-PP formalism.

State-Specific Coupled-Cluster Methods for Excited States

We reexamine ΔCCSD, a state-specific coupled-cluster (CC) with single and double excitations (CCSD) approach that targets excited states through the utilization of non-Aufbau determinants. This methodology is particularly efficient when dealing with doubly excited states, a domain in which the standard equation-of-motion CCSD (EOM-CCSD) formalism falls short. Our goal here to evaluate the effectiveness of ΔCCSD when applied to other types of excited states, comparing its consistency and accuracy with EOM-CCSD. To this end, we report a benchmark on excitation energies computed with the ΔCCSD and EOM-CCSD methods for a set of molecular excited-state energies that encompasses not only doubly excited states but also doublet-doublet transitions and (singlet and triplet) singly excited states of closed-shell systems. In the latter case, we rely on a minimalist version of multireference CC known as the two-determinant CCSD method to compute the excited states. Our data set, consisting of 276 excited states stemming from the quest database [Véril et al., WIREs Comput. Mol. Sci. 2021, 11, e1517], provides a significant base to draw general conclusions concerning the accuracy of ΔCCSD. Except for the doubly excited states, we found that ΔCCSD underperforms EOM-CCSD. For doublet-doublet transitions, the difference between the mean absolute errors (MAEs) of the two methodologies (of 0.10 and 0.07 eV) is less pronounced than that obtained for singly excited states of closed-shell systems (MAEs of 0.15 and 0.08 eV). This discrepancy is largely attributed to a greater number of excited states in the latter set exhibiting multiconfigurational characters, which are more challenging for ΔCCSD. We also found typically small improvements by employing state-specific optimized orbitals.

Copper complex molecules as dye-sensitizers: Hybrid MetaGGA and standard + van der Waals functionals

This study focuses on the use of Density Functional Theory calculations with two main approaches: computational chemistry and computational physics. The following three cases were considered for the derivation: (I) computational chemistry using the M06 hybrid functional, (II) computational chemistry using the standard PBE functional including vdW interactions, and (III) computational physics using the standard PBE functional including vdW interactions and periodic boundary conditions. Since the approximation using hybrid functionals M06 has been extensively validated, this method was used as a reference. The second and third methods are less expensive, it is ideal for use to extend large systems. From the sensitized molecules are found in the gas phase and include solvent effects through the integral equation formalism polarizable continuum model. In a systematic analysis of 15 Cu complex molecules, a complete characterization for DSSCs has been carried out and molecular geometry, electronic and optical measurements have been reported.

Synthesis, Photophysical Properties, Theoretical Studies, and Living Cancer Cell Imaging Applications of New 7-(Diethylamino)quinolone Chalcones

In this article, three unsymmetrical 7-(diethylamino)quinolone chalcones with D-π-A-D and D-π-A-π-D type push-pull molecular arrangements were synthesized via a Claisen-Schmidt reaction. Using 7-(diethylamino)quinolone and vanillin as electron donor (D) moieties, these were linked together through the α,β-unsaturated carbonyl system acting as a linker and an electron acceptor (A). The photophysical properties were studied, revealing significant Stokes shifts and strong solvatofluorochromism caused by the ICT and TICT behavior produced by the push-pull effect. Moreover, quenching caused by the population of the TICT state in THF-H2O mixtures was observed, and the emission in the solid state evidenced a red shift compared to the emission in solution. These findings were corroborated by density functional theory (DFT) calculations employing the wb97xd/6-311G(d,p) method. The cytotoxic activity of the synthesized compounds was assessed on BHK-21, PC3, and LNCaP cell lines, revealing moderate activity across all compounds. Notably, compound 5b exhibited the highest activity against LNCaP cells, with an LC50 value of 10.89 μM. Furthermore, the compounds were evaluated for their potential as imaging agents in living prostate cells. The results demonstrated their favorable cell permeability and strong emission at 488 nm, positioning them as promising candidates for cancer cell imaging applications.

Aufbau Suppressed Coupled Cluster Theory for Electronically Excited States

We introduce an approach to improve single-reference coupled cluster theory in settings where the Aufbau determinant is absent from or plays only a small role in the true wave function. Using a de-excitation operator that can be efficiently hidden within a similarity transform, we create a coupled cluster wave function in which de-excitations work to suppress the Aufbau determinant and produce wave functions dominated by other determinants. Thanks to an invertible and fully exponential form, the approach is systematically improvable, size consistent, size extensive, and, interestingly, size intensive in a granular way that should make the adoption of some ground state techniques, such as local correlation, relatively straightforward. In this initial study, we apply the general formalism to create a state-specific method for orbital-relaxed, singly excited states. We find that this approach matches the accuracy of similar-cost equation-of-motion methods in valence excitations while offering improved accuracy for charge transfer states. We also find the approach to be more accurate than excited-state-specific perturbation theory in both types of states.

Analytical Model of Optical-Field-Driven Subcycle Electron Tunneling Pulses from Two-Dimensional Materials

We develop analytical models of optical-field-driven electron tunneling from the edge and surface of free-standing two-dimensional (2D) materials. We discover a universal scaling between the tunneling current density (J) and the electric field near the barrier (F): In(J/|F|β) ∝ 1/|F| with β values of 3/2 and 1 for edge emission and vertical surface emission, respectively. At ultrahigh values of F, the current density exhibits an unexpected high-field saturation effect due to the reduced dimensionality of the 2D material, which is absent in the traditional bulk material. Our calculation reveals the dc bias as an efficient method for modulating the optical-field tunneling subcycle emission characteristics. Importantly, our model is in excellent agreement with a recent experiment on graphene. Our results offer a useful framework for understanding optical-field tunneling emission from 2D materials, which are helpful for the development of optoelectronics and emerging petahertz vacuum nanoelectronics.

Theoretical explanation of direct photolysis and indirect photolysis of bendazone with •OH, •SO4-, and •CO3- in water: mechanism insights and ecotoxicity evaluation

Bendazone (BNTE) is an herbicide and a highly concerned pollutant in aquatic environments. Understanding the photochemical behavior of BNTE in water is crucial for evaluating its photochemical conversion process in aquatic environments. This study analyzed the direct photolysis and indirect photolysis pathways of two dissociated forms of BNTE in water through density functional theory and time-dependent density functional theory method. The results show that the reaction types of indirect photolysis of BNTE with free radicals (•OH, •SO4-, and •CO3-) are OH- addition, SO4- addition, and CO3- addition. In the process of indirect photolysis of BNTE and free radicals, the photolysis of •OH and BNTE was the easiest, followed by •SO4-. In addition, the active site of BNTE reacting with •OH is C8, and the active site of BNTE reacting with •SO4- is C10. However, the photolysis effect of •CO3- on BNTE is very small, indicating that •CO3- in water plays a secondary role in the indirect photolysis of BNTE. In the direct photolysis of BNTE, N1-C6 bond breaking is difficult to occur spontaneously in the environment due to its high endothermic property and energy barrier. The direct photolysis pathway of BNTE involves the break of the N1-S2/S2-N3/N3-C12 bond. In addition, the ecological toxicity evaluation showed that toxicity of most of the degradation products were reduced, but the toxicity level was still maintained at a harmful level. Our findings provide the photochemical fate of BNTE in aquatic environments and will help to more accurately understand their photochemical conversion mechanisms in the environment.

Real-Time Time-Dependent Density Functional Theory for Simulating Nonequilibrium Electron Dynamics

The explicit real-time propagation approach for time-dependent density functional theory (RT-TDDFT) has increasingly become a popular first-principles computational method for modeling various time-dependent electronic properties of complex chemical systems. In this Perspective, we provide a nontechnical discussion of how this first-principles simulation approach has been used to gain novel physical insights into nonequilibrium electron dynamics phenomena in recent years. Following a concise overview of the RT-TDDFT methodology from a practical standpoint, we discuss our recent studies on the electronic stopping of DNA in water and the Floquet topological phase as examples. Our discussion focuses on how RT-TDDFT simulations played a unique role in deriving new scientific understandings. We then discuss existing challenges and some new advances at the frontier of RT-TDDFT method development for studying increasingly complex dynamic phenomena and systems.

What Happens to a Pyrrole Hemithioindigo Photoswitch Trapped in a Fluorescent Protein?

Artificial molecular photoswitches that can be reversibly controlled into different configurations by external light stimulation have broad application prospects in various fields, such as materials and chemical biology. Among them, the pyrrole hemithioindigo (PHT) photoswitch has a geometric structure similar to that of the fluorescent protein chromophore. What happens when the chromophore is replaced by PHT, and does it achieve similar functions to the original one? To answer these questions, we carried out ONIOM(QM/MM) and classical molecular dynamics studies on the photoisomerization mechanism and spectroscopic properties of PHT in the fluorescent protein. The results showed that in the protein environment, the fate of excited PHT is governed by the competition between fluorescence emission and hula-twist isomerization. Due to the strong steric hindrance effects caused by the interlacing residues in the protein that restrict the PHT conformation transformation, the cis-to-trans isomerization process of PHT needs to overcome a barrier of at least 4.9 kcal/mol; thus, fluorescence emission is more dominant in competition. It is also found that the intermolecular interaction between LYS67 and the carbonyl oxygen of PHT has a significant effect on the fluorescence emission. These results revealed the photochemical reaction mechanism of a light-driven molecular switch in the fluorescent protein and provided further theoretical support for the field of chemical biology.

Chemical Investigation on the Mechanism and Kinetics of the Atmospheric Degradation Reaction of Trichlorofluoroethene by OH⋅ and Its Subsequent Fate in the Presence of O2 /NOx

The M06-2X/6-311++G(d,p) level of theory was used to examine the degradation of Trichlorofluoroethene (TCFE) initiated by OH⋅ radicals. Additionally, the coupled-cluster single-double with triple perturbative [CCSD(T)] method was employed to refine the single-point energies using the complete basis set extrapolation approach. The results indicated that OH-addition is the dominant pathway. OH⋅ adds to both the C1 and C2 carbons, resulting in the formation of the C(OH)Cl2 -⋅CClF and ⋅CCl2 -C(OH)ClF species. The associated barrier heights were determined to be 1.11 and -0.99 kcal mol-1 , respectively. Furthermore, the energetic and thermodynamic parameters show that pathway 1 exhibits greater exothermicity and exergonicity compared to pathway 2, with differences of 8.11 and 8.21 kcal mol-1 , correspondingly. The primary pathway involves OH addition to the C2 position, with a rate constant of 6.2×10-13  cm3 molecule-1 sec-1 at 298 K. This analysis served to estimate the atmospheric lifetime, along with the photochemical ozone creation potential (POCP) and ozone depletion potential (ODP). It yielded an atmospheric lifetime of 8.49 days, an ODP of 4.8×10-4 , and a POCP value of 2.99, respectively. Radiative forcing efficiencies were also estimated at the M06-2X/6-311++G(d,p) level. Global warming potentials (GWPs) were calculated for 20, 100, and 500 years, resulting in values of 9.61, 2.61, and 0.74, respectively. TCFE is not expected to make a significant contribution to the radiative forcing of climate change. The results obtained from the time-dependent density functional theory (TDDFT) indicated that TCFE and its energized adducts are unable to photolysis under sunlight in the UV and visible spectrum. Secondary reactions involve the [TCFE-OH-O2 ]⋅ peroxy radical, leading subsequently to the [TCFE-OH-O]⋅ alkoxy radical. It was found that the alkoxy radical resulting from the peroxy radical can lead to the formation of phosgene (COCl2 ) and carbonyl chloride fluoride (CClFO), with phosgene being the primary product.

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Coordinate Descent Full Configuration Interaction for Excited States

An efficient excited state method, named xCDFCI, in the configuration interaction framework is proposed. xCDFCI extends the unconstrained nonconvex optimization problem in coordinate descent full configuration interaction (CDFCI) to a multicolumn version for low-lying excited states computation. The optimization problem is addressed via a tailored coordinate descent method. In each iteration, a determinant is selected based on an approximated gradient, and coefficients of all states associated with the selected determinant are updated. A deterministic compression is applied to limit memory usage. We test xCDFCI applied to H2O and N2 molecules under the cc-pVDZ basis set. For both systems, five low-lying excited states in the same symmetry sector are calculated, together with the ground state. xCDFCI also produces accurate binding curves of the carbon dimer in the cc-pVDZ basis with chemical accuracy, where the ground state and four excited states in the same symmetry sector are benchmarked.

Exact Excited-State Functionals of the Asymmetric Hubbard Dimer

The exact functionals associated with the (singlet) ground state and the two singlet excited states of the asymmetric Hubbard dimer at half-filling are calculated using both Levy's constrained search and Lieb's convex formulation. While the ground-state functional is, as is commonly known, a convex function with respect to the density, the functional associated with the doubly excited state is found to be concave. Also, because the density-potential mapping associated with the first excited state is noninvertible, its "functional" is a partial, multivalued function composed of one concave and one convex branch that correspond to two separate domains of the external potential. Remarkably, it is found that, although the one-to-one mapping between density and external potential may not apply (as in the case of the first excited state), each state-specific energy and corresponding universal functional are "functions" whose derivatives are each other's inverse, just as in the ground state formalism.

Ultrafast non-adiabatic dynamics of stilbene-based plant-derived sunscreens with cis-trans isomerization structures

In this work, we investigated the potential UV protection mechanism of the natural compounds hydroxy resveratrol and pterostilbene by combining theoretical calculations and femtosecond transient absorption spectra (FTAS). The UV absorption spectra showed that the two compounds exhibited strong absorption properties and high photostability. We found two molecules will reach the S₁ state or an even higher excited state after UV exposure and molecules in S₁ will cross a lower energy barrier to reach the conical intersection. The adiabatic trans-cis isomerization process happened and finally return to the ground. Meanwhile, FTAS clarified the time scale of trans-cis isomerization of two molecules was ∼ 10 ps, which also met the requirement of fast energy relaxation. This work also provides theoretical guidance for developing new sunscreen molecules from natural stilbene.

Excited-State-Specific Pseudoprojected Coupled-Cluster Theory

We present an excited-state-specific coupled-cluster approach in which both the molecular orbitals and cluster amplitudes are optimized for an individual excited state. The theory is formulated via a pseudoprojection of the traditional coupled-cluster wavefunction that allows correlation effects to be introduced atop an excited-state mean field starting point. The approach shares much in common with ground-state CCSD, including size extensivity and an N6 cost scaling. Preliminary numerical tests show that, when augmented with N5 cost perturbative corrections for key terms, the method can improve over excited-state-specific second-order perturbation theory in valence, charge transfer, and Rydberg states.

Density functional applications of jellium with a local gap model correlation energy functional

We develop a realistic density functional approximation for the local gap, which is based on a semilocal indicator that shows good screening properties. The local band model has remarkable density scaling behaviors and works properly for the helium isoelectronic series for the atoms of the Periodic Table, as well as for the non-relativistic noble atom series (up to 2022 e-). Due to these desirable properties, we implement the local gap model in the jellium-with-gap correlation energy, developing the local-density-approximation-with-gap correlation functional (named LDAg) that correctly gives correlation energies of atoms comparable with the LDA ones but shows an improvement for ionization potential of atoms and molecules. Thus, LDAg seems to be an interesting and useful tool in density functional theory.

Novel Triangulenes: Computational Investigations of Energy Thresholds for Photocatalytic Water Splitting

Organic materials with Inverted Singlet-Triplet (INVEST) gaps are interesting for their potential use in photocatalytic small molecule transformations such as the entirely solar-driven water splitting reaction. However, only a few INVEST emitters are thermodynamically able to split water requiring a first singlet excited dark state, S1 , above 1.27 or 1.76 eV, and absorption near solar the maximum, 2.57 eV. These requirements and the INVEST character are key for achieving a long-lived photocatalyst for water splitting. The only known INVEST emitters that conform to these criteria are large triangular boron carbon nitrides with unknown synthesis pathways. Using ADC(2), a quantum-mechanical method, we describe three triangulenes. 3 a is a cyano azacyclopenta[cd]phenalene derivative while 3 b and 3 c are cycl[3.3.3]azine derivatives. 3 b has a previously undescribed disulfide bridge. Overall, 3 a fulfills requirements for photocatalytic four-electron reduction of water while the S1 states of 3 b and 3 c are likely slightly low for the two-electron reduction process. By analyzing impacts of ligands, we find that there are guidelines describing how S1 -S5 energies and oscillator strengths, T1 energies, and ΔES1T1 gaps are affected, requiring deep-learning algorithms for which studies will be presented by us in due time. The impact of ground-state geometries, solvation effects, as well as reduced-cost ADC(2) algorithms on our findings are also discussed.

Better performance of Hartree-Fock over DFT: a quantum mechanical investigation on pyridinium benzimidazolate types of zwitterions in the light of localization/delocalization issues

CONTEXT

With the advent of fast computing facilities, combined with rapid emerges of many new and intricate quantum mechanical functionals, computations with pure Hartree-Fock (HF) theory are now-a-days regarded as trivial or obsolete, or even considered as not reliable by many researchers. Consequently, current trends in computational chemistry show extensive use of post-HF theories for smaller molecular systems and various DFT methods for organic and inorganic chemistry related problems (larger molecules/systems). In this contribution, I have tried to show that sometimes, HF might be more suitable over DFT methodologies in addressing structure-property correlations. Molecules studied here were previously synthesized by Boyd in 1966 and important experimental data were produced by Alcalde and co-workers in 1987. Comparison of computed and experimental results clearly shows that HF method was more effective in reproducing the experimental data compared to especially the DFT methodologies. Reliability of HF method was further assured from the very similar results shown by the CCSD, CASSCF, CISD and QCISD methods. Current study also indicates that the localization issue associated with HF proved to be advantageous over delocalization issue of DFT based methodologies, in correctly describing the structure-property correlation for zwitterion systems.

METHODS

All computations were performed with Gaussian 09. A wide-range of quantum mechanical methodologies, HF, B3LYP, CAM-B3LYP, BMK, B3PW91, TPSSh, LC-ωPBE, M06-2X, M06-HF, ωB97xD, MP2, CASSCF, CCSD, QCISD, CISD and semi-empirical methods like, Huckel, CNDO, AM1, PM3MM and PM6, were used for investigations.

Theoretical Insights into Aggregation-Induced Emission with the Ionic π Fluorophore: The Importance of Choosing the Dimer QM Model in the ONIOM Study

In understanding the mechanism of aggregation-induced emission (AIE), the multilevel ONIOM framework has been demonstrated as one of the efficient tools that can capture the essential mechanistic information by choosing a single fluorophore as the quantum mechanics (QM) model and putting all surrounding molecules in the low-level region. Recently, the ionic styryl-pyridine salt (namely, SPH) has been reported as a new class of AIEgen with a high fluorescence yield. In the SPH crystal, a pair of ionic SPH molecules are closely stacked with each other in an antiparallel, head-to-tail pattern, thus the choice of QM models (an individual or dimeric structure) becomes critical in the ONIOM study. Herein we report the AIE mechanism of the ionic SPH at the QM ((TD)-CAM-B3LYP) and ONIOM(QM:MM) levels. As usual, the fluorescence quenching of SPH in tetrahydrofuran (THF) solution is attributed to a nonradiative relaxation via the central C═C bond rotation, with a rather low barrier of 2.7 kcal/mol. In crystals, either with a monomer or dimer model, the fluorescence quenching channel is found to be restricted due to the obvious C═C rotation barriers. Compared with the monomer model, the dimer model, by treating the orbital interaction of the two SPH molecules at the QM level, provides significantly increased barriers and a red-shifted emission wavelength that better matches the experimental value. In addition, the calculated exciton coupling in the fluorescence emission state can be discovered only by a dimer model. The findings here emphasize not only the importance of choosing a proper model in the ONIOM study of AIE but also expanding our understanding of novel AIE systems.

Stochastic Real-Time Second-Order Green's Function Theory for Neutral Excitations in Molecules and Nanostructures

We present a real-time second-order Green's function (GF) method for computing excited states in molecules and nanostructures, with a computational scaling of O(Ne3), where Ne is the number of electrons. The cubic scaling is achieved by adopting the stochastic resolution of the identity to decouple the 4-index electron repulsion integrals. To improve the time propagation and the spectral resolution, we adopt the dynamic mode decomposition technique and assess the accuracy and efficiency of the combined approach for a chain of hydrogen dimer molecules of different lengths. We find that the stochastic implementation accurately reproduces the deterministic results for the electronic dynamics and excitation energies. Furthermore, we provide a detailed analysis of the statistical errors, bias, and long-time extrapolation. Overall, the approach offers an efficient route to investigate excited states in extended systems with open or closed boundary conditions.

Real-Time GW-Ehrenfest-Fan-Migdal Method for Nonequilibrium 2D Materials

Quantum simulations of photoexcited low-dimensional systems are pivotal for understanding how to functionalize and integrate novel two-dimensional (2D) materials in next-generation optoelectronic devices. First-principles predictions are extremely challenging due to the simultaneous interplay of light-matter, electron-electron, and electron-nuclear interactions. We here present an advanced ab initio many-body method that accounts for quantum coherence and non-Markovian effects while treating electrons and nuclei on equal footing, thereby preserving fundamental conservation laws like the total energy. The impact of this advancement is demonstrated through real-time simulations of the complex multivalley dynamics in a molybdenum disulfide (MoS2) monolayer pumped above gap. Within a single framework, we provide a parameter-free description of the coherent-to-incoherent crossover, elucidating the role of microscopic and collective excitations in the dephasing and thermalization processes.

Synthesis, Characterization, Fluorescence Properties, and DFT Modeling of Difluoroboron Biindolediketonates

We report a simple and efficient strategy to enhance the fluorescence of biocompatible biindole diketonates (bdks) in the visible spectrum through difluoroboronation (BF₂bdks complexes). Emission spectroscopy testifies an increase in the fluorescence quantum yields from a few percent to as much as >0.7. This massive increment is essentially independent of substitutions at the indole (-H, -Cl, and -OCH₃) and corresponds to a significant stabilization of the excited state with respect to non-radiative decay mechanisms: the non-radiative decay rates are reduced by as much as an order of magnitude, from 10⁹ s^-1 to 10⁸ s^-1, upon difluoroboronation. The stabilization of the excited state is large enough to enable sizeable ¹O₂ photosensitized production. Different time-dependent (TD) density functional theory (DFT) methods were assessed in their ability to model the electronic properties of the compounds, with TD-B3LYP-D3 providing the most accurate excitation energies. The calculations associate the first active optical transition in both the bdks and BF₂bdks electronic spectra to the S₀ → S₁ transition, corresponding to a shift in the electronic density from the indoles to the oxygens or the O-BF₂-O unit, respectively.

Computational Study of Photodegradation Process and Conversion Products of the Antidepressant Citalopram in Water

Citalopram (CIT) is a commonly prescribed medication for depression. However, the photodegradation mechanism of CIT has not yet been fully analyzed. Therefore, the photodegradation process of CIT in water is studied by density functional theory and time-dependent density functional theory. The calculated results show that during the indirect photodegradation process, the indirect photodegradation of CIT with ·OH occurs via OH-addition and F-substitution. The minimum activation energy of C10 site was 0.4 kcal/mol. All OH-addition and F-substitution reactions are exothermic. The reaction of ¹O₂ with CIT includes the substitution of ¹O₂ for F and an addition reaction at the C14 site. The E_a value of this process is 1.7 kcal/mol, which is the lowest activation energy required for the reaction of ¹O₂ with CIT. C-C/C-N/C-F cleavage is involved in the direct photodegradation process. In the direct photodegradation of CIT, the activation energy of the C7-C16 cleavage reaction was the lowest, which was 12.5 kcal/mol. Analysis of the E_a values found that OH-addition and F-substitution, the substitution of ¹O₂ for F and addition at the C14 site, as well as the cleavage reactions of C6-F/C7-C16/C17-C18/C18-N/C19-N/C20-N are the main pathways of photodegradation of CIT.

Photodegradation fate of different dissociation species of antidepressant paroxetine and the effects of metal ion Mg2+: Theoretical basis for direct and indirect photolysis

Paroxetine (abbreviated as PXT) has been widely used as one of the standard antidepressants for the treatment of depression. PXT has been detected in the aqueous environment. However, the photodegradation mechanism of PXT remains unclear. The present study aimed to use density functional theory and time-dependent density functional theory to study the photodegradation process of two dissociated forms of PXT in water. The main mechanisms include direct and indirect photodegradation via reaction with ·OH and ¹O₂ and photodegradation mediated by the metal ion Mg²⁺. Based on the calculations, PXT and PXT-Mg²⁺ complexes in water are photodegraded mainly indirectly and directly. It was found that PXT and PXT-Mg²⁺ complexes were photodegraded by H-abstraction, OH-addition and F-substitution. The main reaction of PXT indirect photolysis is OH-addition reaction, while the main reaction of PXT⁰-Mg²⁺ complex is H-abstraction. All the reaction pathways of H-abstraction, OH-addition and F-substitution are exothermic. PXT⁰ reacts more readily with ·OH or ¹O₂ in water than PXT⁺. However, the higher activation energy of PXT with ¹O₂ indicates that the ¹O₂ reaction plays a minor role in the photodegradation pathway. The direct photolysis process of PXT includes ether bond cleavage, defluorination, and dioxolane ring-opening reaction. In the PXT-Mg²⁺ complex, the direct photolysis process occurs via a dioxolane ring opening. Additionally, Mg²⁺ in water has a dual effect on the direct and indirect photolysis of PXT. In other words, Mg²⁺ can inhibit or promote their photolytic reactions. Overall, PXT in natural water mainly undergo direct and indirect photolysis reactions with ·OH. The main products include direct photodegradation products, hydroxyl addition products and F-substitution products. These findings provide critical information for predicting the environmental behavior and transformation of antidepressants.

Polarization Dependence of Laser-Induced Dynamics on Non-Flat Metal Surfaces: A Time-Dependent Density Functional Theory Approach

Real-time time-dependent density functional theory was used to study the laser-pulse-induced ion dynamics on metal surfaces featuring rows of atomic ridges. In contrast to atomically flat surfaces, the rows of atomic ridges induce anisotropy on the surface even in surface-parallel directions. This anisotropy causes the laser-induced ion dynamics to depend on the orientation of the laser polarization vector in the surface-parallel directions. This polarization dependence occurs for both copper (111) and aluminum (111) surfaces, indicating that the existence of localized d orbitals in the electronic system does not play a crucial role. The difference in kinetic energies between ions on the ridges and those on the planar surface reached a maximum when the laser polarization vector was perpendicular to the rows of ridges but parallel to the surface. A simple mechanism for the polarization dependence and some potential applications in laser processing are discussed.

Excited States, Symmetry Breaking, and Unphysical Solutions in State-Specific CASSCF Theory

State-specific electronic structure theory provides a route toward balanced excited-state wave functions by exploiting higher-energy stationary points of the electronic energy. Multiconfigurational wave function approximations can describe both closed- and open-shell excited states and avoid the issues associated with state-averaged approaches. We investigate the existence of higher-energy solutions in complete active space self-consistent field (CASSCF) theory and characterize their topological properties. We demonstrate that state-specific approximations can provide accurate higher-energy excited states in H₂ (6-31G) with more compact active spaces than would be required in a state-averaged formalism. We then elucidate the unphysical stationary points, demonstrating that they arise from redundant orbitals when the active space is too large or symmetry breaking when the active space is too small. Furthermore, we investigate the singlet-triplet crossing in CH₂ (6-31G) and the avoided crossing in LiF (6-31G), revealing the severity of root flipping and demonstrating that state-specific solutions can behave quasi-diabatically or adiabatically. These results elucidate the complexity of the CASSCF energy landscape, highlighting the advantages and challenges of practical state-specific calculations.

A theoretical study of the time-resolved x-ray absorption spectrum of the photoionized BT-1T cation

The time-resolved x-ray absorption spectrum of the BT-1T cation (BT-1T+) is theoretically simulated in order to investigate the charge transfer reaction of the system. We employ both trajectory surface hopping and quantum dynamics to simulate the structural evolution over time and the changes in the state populations. To compute the static x-ray absorption spectra (XAS) of the ground and excited states, we apply both the time-dependent density functional theory and the coupled cluster singles and doubles method. The results obtained are in good agreement between the methods. It is, furthermore, found that the small structural changes that occur during the reaction have little effect on the static XAS. Hence, the tr-XAS can be computed based on the state populations determined from a nuclear dynamics simulation and one set of static XAS calculations, utilizing the ground state optimized geometry. This approach can save considerable computational resources, as the static spectra need not to be calculated for all geometries. As BT-1T is a relatively rigid molecule, the outlined approach should only be considered when investigating non-radiative decay processes in the vicinity of the Franck-Condon point.

Efficient Synthesis, Spectroscopic Characterization, and Nonlinear Optical Properties of Novel Salicylaldehyde-Based Thiosemicarbazones: Experimental and Theoretical Studies

Currently, we reported the synthesis of six novel salicylaldehyde-based thiosemicarbazones (BHCT1-HBCT6) via condensation of salicylaldehyde with respective thiosemicarbazide. Through various spectroscopic methods, UV-visible and NMR, the chemical structures of BHCT1-HBCT6 compounds were determined. Along with synthesis, a computational study was also performed at the M06/6-31G(d,p) functional. Various analyses such as natural bond orbital (NBO) analysis, natural population analysis, frontier molecular orbital (FMO) analysis, and molecular electrostatic potential surfaces were carried out to understand the nonlinear optical (NLO) characteristics of the synthesized compounds. Additionally, a comparative study was carried out between DFT and experimental results (UV-vis study), and a good agreement was observed in the results. The energy gap calculated through FMOs was found to be in decreasing order as 4.505 (FHCT2) > 4.499 (HBCT6) > 4.497 (BHCT1) = 4.497(HMCT5) > 4.386 (CHCT3) > 4.241(AHCT4) in eV. The global reactivity parameters (GRPs) were attained through E _HOMO and E _LUMO, which described the stability and hardness of novel compounds. The NBO approach confirmed the charge delocalization and stability of the molecules. Among all the investigated compounds, a larger value (557.085 a.u.) of first hyperpolarizability (β_tot) was possessed by CHCT3. The NLO response (β_tot) of BHCT1-HBCT6 was found to be 9.145, 9.33, 13.33, 5.43, 5.68, and 10.13 a.u. times larger than that of the standard para-nitroaniline molecule. These findings ascertained the potential of entitled ligands as best NLO materials for a variety of applications in modern technology.

State-Specific Configuration Interaction for Excited States

We introduce and benchmark a systematically improvable route for excited-state calculations, labeled state-specific configuration interaction (ΔCI), which is a particular realization of multiconfigurational self-consistent field and multireference configuration interaction. Starting with a reference built from optimized configuration state functions, separate CI calculations are performed for each targeted state (hence, state-specific orbitals and determinants). Accounting for single and double excitations produces the ΔCISD model, which can be improved with second-order Epstein-Nesbet perturbation theory (ΔCISD+EN2) or a posteriori Davidson corrections (ΔCISD+Q). These models were gauged against a vast and diverse set of 294 reference excitation energies. We have found that ΔCI is significantly more accurate than standard ground-state-based CI, whereas close performances were found between ΔCISD and EOM-CC2 and between ΔCISD+EN2 and EOM-CCSD. For larger systems, ΔCISD+Q delivers more accurate results than EOM-CC2 and EOM-CCSD. The ΔCI route can handle challenging multireference problems, singly and doubly excited states, from closed- and open-shell species, with overall comparable accuracy and thus represents a promising alternative to more established methodologies. In its current form, however, it is reliable only for relatively low-lying excited states.

Thematic Exordium for Special Issue "Density Functional Theory Application on Chemical Calculation"

The history of quantum chemistry dates back to 1926, when the German physicist Erwin Schrödinger, in his classical works [...].

Noncollinear and Spin-Flip TDDFT in Multicollinear Approach

Time-dependent density functional theory (TDDFT) is one of the most important tools for investigating the excited states of electrons. The TDDFT calculation for spin-conserving excitation, where collinear functionals are sufficient, has obtained great success and has become routine. However, TDDFT for noncollinear and spin-flip excitations, where noncollinear functionals are needed, is less widespread and still a challenge nowadays. This challenge lies in the severe numerical instabilities that root in the second-order derivatives of commonly used noncollinear functionals. To be free from this problem radically, noncollinear functionals with numerical stable derivatives are desired, and our recently developed approach, called the multicollinear approach, provides an option. In this work, the multicollinear approach is implemented in noncollinear and spin-flip TDDFT, and prototypical tests are given.

Role of anchoring groups on the light harvesting and optoelectronic properties of triphenylamine derivatives: insights from theory

BACKGROUND

In the present work, DFT and time-dependent DFT calculations were performed to investigate the role of anchoring groups on the photophysical properties and reveal structure-property correlations of triphenylamine (TPA) derivatives. The selected anchoring groups are tetrazole, acrylamide, hydantoin, and rhodanine.

RESULTS

Our results show that the different anchoring groups employed alter the planarity, intramolecular charge transfer properties, and HOMO-LUMO gap and hence influence the optoelectronic properties of the dyes. Although all molecules fulfill the basic requirements with suitable energy levels, band gap, absorption, and charge transfer properties, the dye with rhodanine acceptor (TPA4) was the most promising candidate due to its lowest HOMO-LUMO gap, red-shifted highest λ_max absorption value, better ICT pattern, low total reorganization energy, and good electron injection properties. Overall, it is anticipated that the results of this investigation will point to new avenues for the experimental fabrication of remarkably effective metal-free organic dyes for solar cell applications.