Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction

On the third-order nonlinear optical responses of cis and trans stilbenes - a quantum chemistry investigation

The second hyperpolarizabilities (γ) of the stilbene molecular switch in its trans and cis forms have been calculated using quantum chemistry methods to address their third-order nonlinear optical contrasts, to assess the reliability of lower-cost DFT methods, and to make comparisons with experiments. First, the reference CCSD(T) method shows that trans-stilbene presents a γ‖ value twice larger than its cis isomer (its γTHS value is 2.7 times larger). Among more cost-effective methods, reliable results are obtained at MP2 as well as with DFT, provided the CAM-B3LYP or ωB97X-D XCFs are employed. Supplementary DFT calculations have investigated the relationships between the accuracy of the exchange-correlation functionals, the fulfillment of Koopmans' theorem, and the delocalization error, and they demonstrated that satisfying Koopmans' theorem is not the condition for the best accuracy but that functionals with small delocalization errors are generally efficient. Using the selected CAM-B3LYP, large γ enhancements by about 70% (trans-stilbene) and 50% (cis-stilbene) have been evidenced when accounting for solvent effects using an implicit solvation model (IEFPCM), even for apolar solvents. Then, the frequency dispersion of the γ responses has been described using Bishop polynomial expansions, allowing comparisons with a broad set of experimental data. To a certain extent, no systematic agreement between the calculations and the measured values was found. On the one hand, the agreement is satisfactory for the γ(-ω;ω,-ω,ω) quantities, provided that the dominant vibrational contribution is taken into account. On the other hand, the agreement is poor for the γ(-2ω;ω,ω,0) and γ(-3ω;ω,ω,ω) quantities, while some inconsistencies between experimental values are also highlighted.

Multifaceted Study of a Y-Shaped Pyrimidine Compound: Assessing Structural Properties, Docking Interactions, and Third-Order Nonlinear Optics

In this study, we report the synthesis of a new compound, N4,N4-dimethyl-2-(methylsulfanyl)-N6-(4-phenoxyphenyl)pyrimidine-4,6-diamine (DMS), and its comprehensive analysis through structural and spectroscopic characterizations, reactivity parameters, and nonlinear optical properties, utilizing a combination of experimental and computational techniques. The experimental aspect of the investigation encompassed structural characterization using X-ray diffraction and spectroscopic assessments employing Fourier-transform infrared, Raman, and nuclear magnetic resonance techniques, along with thermal analysis. Our computational approach involved density functional theory (DFT) calculations and molecular dynamics (MD) simulations to examine the local reactivity properties of DMS. We employed fundamental reactivity descriptors to evaluate DMS's local reactivity and utilized MD simulations to identify DMS atoms engaging in significant interactions with water molecules. We conducted periodic DFT calculations on DMS's crystal structure to investigate the contributions of specific atoms and groups to the compound's overall stability as well as to analyze noncovalent interactions between DMS molecules. We assessed the nonlinear optical properties through dynamic second hyperpolarizability and third-order nonlinear susceptibility calculations. Additionally, we conducted a comparative analysis of the static and dynamic second hyperpolarizability for the DMS molecule within the sum-over-states framework. The obtained value for the third-order nonlinear susceptibility, (λ = 1907 nm), exceeds those of other organic materials reported in previous studies, indicating that the DMS crystal holds promise as a nonlinear optical material for potential application in photonic device fabrication. Furthermore, molecular docking studies were performed with the 3E5A, 4EUT, and 4EUU proteins, yielding binding affinities of -8.1, -8.2, and -8.3 kcal/mol, respectively, in association with the ligand.

A quantum chemical investigation of the second hyperpolarizability of p-nitroaniline

Recent measurements of the third harmonic scattering responses of molecules have given a new impetus for computing molecular second hyperpolarizabilities (γ) and for deducing structure-property relationships. This paper has employed a variety of wavefunction and density functional theory methods to evaluate the second hyperpolarizability of the p-nitroaniline prototypical push-pull π-conjugated molecule, addressing also numerical aspects, such as the selection of an integration grid and the impact of the order of differentiation vs the achievable accuracy by using the Romberg quadrature. The reliability of the different methods has been assessed by comparison to reference Coupled-Cluster Singles and Doubles with perturbative treatment of the Triples results. On the one hand, among wavefunction methods, the MP2 scheme offers the best accuracy/cost ratio for computing the static γ. On the other hand, using density functional theory, γ remains a challenging property to compute because all conventional, global hybrid or range-separated hybrid, exchange-correlation functionals underestimate static γ values by at least 15%. Even tuning the range-separating parameter to minimize the delocalization errors does not enable to improve the γ values. Nevertheless, the original double-hybrid B2-PLYP functional, which benefits from 27% of PT2 correlation and 53% Hartree-Fock exchange, provides accurate estimates of static γ values. Unfortunately, the best performing exchange-correlation functionals for γ are not necessarily reliable for the first hyperpolarizability, β, and vice versa. In fact, the β of p-nitroaniline (pNA) could be predicted, with a good accuracy, with several hybrid exchange-correlation functionals (including by tuning the range-separating parameter), but these systematically underestimate γ. As for γ, the MP2 wavefunction method remains the best compromise to evaluate the first hyperpolarizability of pNA at low computational cost.

Towards high-resolution X-ray scattering as a probe of electron correlation

We demonstrate that X-ray scattering can be used as a probe of electron–electron correlation.

Second-order nonlinear optical properties of two chalcone derivatives: insights from sum-over-states

In this study, a combined experimental and theoretical study of the nonlinear optical properties (NLO) of two chalcone derivatives, (E)-3-(2-methoxyphenyl)-1-(2-(phenylsulfonylamine)phenyl)prop-2-en-1-one (MPSP) and (E)-3-(3-nitrophenyl)-1-(2-(phenylsulfonylamine)phenyl)prop-2-en-1-one (NPSP), in DMSO is reported. The single crystal structures of the compounds, which differ only by the type and position of one substituent, were grown using the slow evaporation technique, and the main structural differences are discussed. The two-photon absorption and first-order hyperpolarizability measurements were performed via the Z-scan technique and hyper-Rayleigh scattering experiment in DMSO. The theoretical calculations were performed using the Density Functional Theory (DFT) at the CAM-B3LYP/6-311++G(d,p) level, and the sum-over-states (SOS) approach in both static and dynamic cases. Besides the electron conjugation achieved by the aromatic rings, olefins, and carbonyl groups, both compounds have a nearly flat chalcone backbone, which is believed to contribute to the nonlinear optical properties. MPSP and NPSP have different positions, even though they have roughly the same conformation and form C-HO interactions. For several studied frequencies, the HRS first hyperpolarizability values for MPSP are greater than those for NPSP, indicating that in most cases the NLO properties of MPSP are better. The comparison among the theoretical and experimental HRS first hyperpolarizability results showed a good agreement. In addition, the two-dimensional second order nonlinear optical spectra obtained from the sum-over-states model indicate good second-order NLO responses of the two chalcone derivatives under external fields. Our findings are important not only to show the potential nonlinear optical application of the two new compounds but also to gain an insight into how different chemical compositions might affect the crystal structures and physico-chemical properties.

A new isostructural halogenated chalcone with optical properties

Chalcones are organic compounds that present a number of properties. This study presents a comprehensive structural description of a new derivative of a chlorine-substituted chalcone in comparison with a bromine chalcone. Also, supermolecule and sum-over-state approach were used to describe the optical properties of these structures regarding the substitution of the bromine by the chlorine atom. In addition, the electrical properties, dipole moment, linear polarizability, and second IDRI hyperpolarizability were calculated. The linear refractive index and the third-order nonlinear macroscopic susceptibility were evaluated as a function of the applied electric field frequency. Furthermore, the quantum mechanics calculations that were implemented at the M06-2X/6-311++G(d,p) level of the theory for these isostructural chalcones indicate that the change in halogen atoms does not cause meaningful changes in their conformation. Finally, we can postulate that side-to-side and the antiparallel interactions are the interaction forces that drive the crystal growth for new isostructural chalcones. The NLO properties showed title compounds that are good candidates for use as NLO materials.

Investigation of challenging spin systems using Monte Carlo configuration interaction and the density matrix renormalization group

We investigate if a range of challenging spin systems can be described sufficiently well using Monte Carlo configuration interaction (MCCI) and the density matrix renormalization group (DMRG) in a way that heads toward a more "black box" approach. Experimental results and other computational methods are used for comparison. The gap between the lowest doublet and quartet state of methylidyne (CH) is first considered. We then look at a range of first-row transition metal monocarbonyls: MCO when M is titanium, vanadium, chromium, or manganese. For these MCO systems we also employ partially spin restricted open-shell coupled-cluster (RCCSD). We finally investigate the high-spin low-lying states of the iron dimer, its cation and its anion. The multireference character of these molecules is also considered. We find that these systems can be computationally challenging with close low-lying states and often multireference character. For this more straightforward application and for the basis sets considered, we generally find qualitative agreement between DMRG and MCCI. © 2017 Wiley Periodicals, Inc.

Challenging compounds for calculating molecular second hyperpolarizabilities: the triplet state of the trimethylenemethane diradical and two derivatives

The second hyperpolarizability γ of trimethylenemethane (TMM) and two 1,3-dipole derivatives (NXA and OXA) in their triplet ground state has been evaluated at the UCCSD(T) level with the d-aug-cc-pVDZ extended basis set, highlighting that γ decreases from TMM to NXA and OXA, following the opposite order of their permanent dipole moments. These results are then used to benchmark a broad range of levels of approximation. So, the UMP2, UMP4, and UCCSD methods can be used to characterize γ of TMM and NXA but not of OXA. In that case, the large field-induced charge transfer contribution is difficult to handle using the MPn methods and only the UCCSD method provides values close to the UCCSD(T) reference. Turning to the performance of DFT with typical exchange-correlation functionals, the UM06-2X functional, which contains 54% of HF exchange, performs very well with a maximum of 4.5% of difference with respect to the reference values. On the other hand, employing less HF exchange leads to an overestimation of the responses whereas range-separated hybrids generally underestimate the second hyperpolarizabilities. Finally, the use of spin-projected methods for these 1,3-dipole triplet molecules has a little impact since the spin contamination is almost negligible.

Investigating Multireference Character and Correlation in Quantum Chemistry

We review a range of multireference diagnostics for quantum chemistry and discuss them in terms of choices of the molecular orbitals. We show how an approach1 of P.-O. Löwdin can also be viewed as quantifying the electron correlation via the spatial entanglement relative to a single determinant. We consider three example systems from quantum chemistry that exhibit three different combinations of multireference character and correlation: not strongly multireference and not strongly correlated, strongly multireference but not strongly correlated, and strongly multireference together with strong correlation. We find that a multireference measure (MR) does not change substantially with the cutoff used for a Monte Carlo configuration interaction calculation and investigate the effect of using natural orbitals. We see that a coupled-cluster singles and doubles diagnostic and a density-functional theory diagnostic give a correct general prediction of the multireference character for these systems. We also look at the issue of multireference character for a collection of noninteracting hydrogen molecules and the effect of basis size on the multireference character of a stretched hydrogen molecule.

The first hyperpolarizability of nitrobenzene in benzene solutions: investigation of the effects of electron correlation within the sequential QM/MM approach

Molecular polarization due to solvation amplifies the impact of electron correlation on the first hyperpolarizability of nitrobenzene.