Quantum mechanical and semiclassical dynamics at a conical intersection

Newton-X Platform: New Software Developments for Surface Hopping and Nuclear Ensembles

Newton-X is an open-source computational platform to perform nonadiabatic molecular dynamics based on surface hopping and spectrum simulations using the nuclear ensemble approach. Both are among the most common methodologies in computational chemistry for photophysical and photochemical investigations. This paper describes the main features of these methods and how they are implemented in Newton-X. It emphasizes the newest developments, including zero-point-energy leakage correction, dynamics on complex-valued potential energy surfaces, dynamics induced by incoherent light, dynamics based on machine-learning potentials, exciton dynamics of multiple chromophores, and supervised and unsupervised machine learning techniques. Newton-X is interfaced with several third-party quantum-chemistry programs, spanning a broad spectrum of electronic structure methods.

A Photochemical Reaction in Different Theoretical Representations

The Born–Oppenheimer picture has forged our representation and interpretation of photochemical processes, from photoexcitation down to the passage through a conical intersection, a funnel connecting different electronic states. In this work, we analyze a full in silico photochemical experiment, from the explicit electronic excitation by a laser pulse to the formation of photoproducts following a nonradiative decay through a conical intersection, by contrasting the picture offered by Born–Oppenheimer and that proposed by the exact factorization. The exact factorization offers an alternative understanding of photochemistry that does not rely on concepts such as electronic states, nonadiabatic couplings, and conical intersections. On the basis of nonadiabatic quantum dynamics performed for a two-state 2D model system, this work allows us to compare Born–Oppenheimer and exact factorization for (i) an explicit photoexcitation with and without the Condon approximation, (ii) the passage of a nuclear wavepacket through a conical intersection, (iii) the formation of excited stationary states in the Franck–Condon region, and (iv) the use of classical and quantum trajectories in the exact factorization picture to capture nonadiabatic processes triggered by a laser pulse.

Quantum transition probabilities due to overlapping electromagnetic pulses: Persistent differences between Dirac's form and nonadiabatic perturbation theory

The probability of transition to an excited state of a quantum system in a time-dependent electromagnetic field determines the energy uptake from the field. The standard expression for the transition probability has been given by Dirac. Landau and Lifshitz suggested, instead, that the adiabatic effects of a perturbation should be excluded from the transition probability, leaving an expression in terms of the nonadiabatic response. In our previous work, we have found that these two approaches yield different results while a perturbing field is acting on the system. Here, we prove, for the first time, that differences between the two approaches may persist after the perturbing fields have been completely turned off. We have designed a pair of overlapping pulses in order to establish the possibility of lasting differences, in a case with dephasing. Our work goes beyond the analysis presented by Landau and Lifshitz, since they considered only linear response and required that a constant perturbation must remain as t → ∞. First, a "plateau" pulse populates an excited rotational state and produces coherences between the ground and excited states. Then, an infrared pulse acts while the electric field of the first pulse is constant, but after dephasing has occurred. The nonadiabatic perturbation theory permits dephasing, but dephasing of the perturbed part of the wave function cannot occur within Dirac's method. When the frequencies in both pulses are on resonance, the lasting differences in the calculated transition probabilities may exceed 35%. The predicted differences are larger for off-resonant perturbations.

A systematic model study quantifying how conical intersection topography modulates photochemical reactions

Despite their important role in photochemistry and expected presence in most polyatomic molecules, conical intersections have been thoroughly characterized in a comparatively small number of systems. Conical intersections can confer molecular photoreactivity or photostability, often with remarkable efficacy, due to their unique structure: at a conical intersection, the adiabatic potential energy surfaces of two or more electronic states are degenerate, enabling ultrafast decay from an excited state without radiative emission, known as nonadiabatic transfer. Furthermore, the precise conical intersection topography determines fundamental properties of photochemical processes, including excited-state decay rate, efficacy, and molecular products that are formed. However, these relationships have yet to be defined comprehensively. In this article, we use an adaptable computational model to investigate a variety of conical intersection topographies, simulate resulting nonadiabatic dynamics, and calculate key photochemical observables. We varied the vibrational mode frequencies to modify conical intersection topography systematically in four primary classes of conical intersections and quantified the resulting rate, total yield, and product yield of nonadiabatic decay. The results reveal that higher vibrational mode frequencies reduce nonadiabatic transfer, but increase the transfer rate and resulting photoproduct formation. These trends can inform progress toward experimental control of photochemical reactions or tuning of molecules' photochemical properties based on conical intersections and their topography.

Ultrafast Dynamics of the Two Lowest Bright Excited States of Cytosine and 1-Methylcytosine: A Quantum Dynamical Study

The nonadiabatic quantum dynamics (QD) of cytosine and 1-methylcytosine in the gas phase is simulated for 250 fs after a photoexcitation to one of the first two bright states. The nuclear wavepacket is propagated on the coupled diabatic potential energy surfaces of the lowest seven excited states, including ππ*, nπ*, and Rydberg states along all the vibrational degrees of freedom. We focus in particular on the interplay between the bright and the dark nπ* states, not considering the decay to the ground electronic state. To run these simulations, we implemented an automatic general procedure to parametrize linear vibronic coupling (LVC) models with time-dependent density functional theory (DFT) computations and interfaced it with Gaussian package. The wavepacket was propagated with the multilayer version of the multiconfigurational time dependent Hartree method. Two different density functionals, PBE0 and CAM-B3LYP, which provide a different description of the relative stability of the lowest energy dark states, were used to parametrize the LVC Hamiltonian. Part of the photoexcited population on lowest HOMO-LUMO transition (π_Hπ_L^*) decays within less than 100 fs to a nπ* state which mainly involves a promotion of an electron from the oxygen lone pair to the LUMO (n_Oπ_L^*). The population of the second ππ* state decays almost completely, in <100 fs, not only to π_Hπ_L^* and to n_Oπ_L^* states but also to another nπ_L^* state involving the nitrogen lone pair. The efficiency of the adopted protocol allowed us to check the accuracy of the predictions by repeating the QD simulations with different LVC Hamiltonians parametrized either at the ground-state minimum or at stationary structures of different relevant excited states.

Photoinduced Double Proton Transfer in the Glyoxal-Methanol Complex Revisited: The Role of the Excited States

Under irradiation in the visible range, the glyoxal-methanol complex in a cryogenic argon matrix undergoes a double proton transfer (DPT) reaction through which the glyoxal molecule isomerizes into hydroxyketene. In this work, we employ electronic structure simulations in order to shed more light on the underlying mechanism. Rewardingly, we find that the lowest singlet excited state (S₁) of the complex acts as a gateway to two previously unknown isomerization pathways, of which one takes place entirely in the singlet manifold and the other also involves the lowest triplet state (T₁). Both of these pathways are fully compatible with the available experimental data, implying that either or both are operative under experimental conditions. In either pathway, the methanol molecule acts as a proton shuttle between the proton-donating and proton-accepting sites of glyoxal, resulting in a dramatic lowering of the potential energy barrier to isomerization with respect to the case of isolated glyoxal. The occurrence of DPT in the singlet manifold is demonstrated directly with the use of nonadiabatic molecular dynamics simulations at the spin-flip time-dependent density functional theory level.

Time-resolved near-edge X-ray absorption fine structure of pyrazine from electronic structure and nuclear wave packet dynamics simulations

As a demonstration of the analysis of the electronic structure and the nuclear dynamics from time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS), we present the TR-NEXAFS spectra of pyrazine following the excitation to the ¹B_2u(ππ^*) state. The spectra are calculated combining the frozen-core/core-valence separated equation-of-motion coupled cluster singles and doubles approach for the spectral signatures and the multiconfiguration time-dependent Hartree method for the wave packet propagation. The population decay from the ¹B_2u(ππ^*) state to the ¹B_3u(nπ^*) and ¹A_u(nπ^*) states, followed by oscillatory flow of population between the ¹B_3u(nπ^*) and ¹A_u(nπ^*) states, is interpreted by means of visualization of the potential energy curves and the reduced nuclear densities. By examining the electronic structure of the three valence-excited states and the final core-excited states, we observe that the population dynamics is explicitly reflected in the TR-NEXAFS spectra, especially when the heteroatoms are selected as the X-ray absorption sites. This work illustrates the feasibility of extracting fine details of molecular photophysical processes from TR-NEXAFS spectra by using currently available theoretical methods.

Performance of trajectory surface hopping method in the treatment of ultrafast intersystem crossing dynamics

We carried out extensive studies to examine the performance of the fewest-switches surface hopping method in the description of the ultrafast intersystem crossing dynamic of various singlet-triplet (S-T) models by comparison with the results of the exact full quantum dynamics. Different implementation details and some derivative approaches were examined. As expected, it is better to perform the trajectory surface hopping calculations in the spin-adiabatic representation or by the local diabatization approach, instead of in the spin-diabatic representation. The surface hopping method provides reasonable results for the short-time dynamics in the S-T model with weak spin-orbital coupling (diabatic coupling), although it does not perform well in the models with strong spin-orbital coupling (diabatic coupling). When the system accesses the S-T potential energy crossing with rather high kinetic energy, the trajectory surface hopping method tends to produce a good description of the nonadiabatic intersystem crossing dynamics. The impact of the decoherence correction on the performance of the trajectory surface hopping is system dependent. It improves the result accuracy in many cases, while its influence may also be minor for other cases.

Nonadiabatic dynamics simulations of singlet fission in 2,5-bis(fluorene-9-ylidene)-2,5-dihydrothiophene crystals

We present simulations of the singlet fission dynamics in 2,5-bis(fluorene-9-ylidene)-2,5-dihydrothiophene (ThBF), a thienoquinoid compound recently investigated experimentally by Kawata et al. The simulation model consisted of two ThBF molecules embedded in their crystal environment. The aim was to understand the singlet fission mechanism, and to predict the excited state lifetimes and the singlet fission quantum yield, hitherto unknown. The simulations were performed by the trajectory surface hopping approach with on-the-fly calculations of the electronic wave functions and energies by the semiempirical FOMO-CI method. We found that the initially photogenerated excitonic bright state decays to the lower dark state with a biexponential behaviour, essentially due to transitions to other close lying states. The dark state in turn decays with a lifetime of about 1 ps to the double triplet 1TT state, which is long-lived, as ascertained by performing a simulation with inclusion of the spin-orbit coupling. The singlet fission quantum yield is predicted to be close to the theoretical maximum of 200%. In view of using this thienoquinoid compound in photovoltaic devices, a major drawback is the low energy of the T1 state at its equilibrium geometry.

Quantum dynamics in open quantum-classical systems

Often quantum systems are not isolated and interactions with their environments must be taken into account. In such open quantum systems these environmental interactions can lead to decoherence and dissipation, which have a marked influence on the properties of the quantum system. In many instances the environment is well-approximated by classical mechanics, so that one is led to consider the dynamics of open quantum-classical systems. Since a full quantum dynamical description of large many-body systems is not currently feasible, mixed quantum-classical methods can provide accurate and computationally tractable ways to follow the dynamics of both the system and its environment. This review focuses on quantum-classical Liouville dynamics, one of several quantum-classical descriptions, and discusses the problems that arise when one attempts to combine quantum and classical mechanics, coherence and decoherence in quantum-classical systems, nonadiabatic dynamics, surface-hopping and mean-field theories and their relation to quantum-classical Liouville dynamics, as well as methods for simulating the dynamics.

Analysis of geometric phase effects in the quantum-classical Liouville formalism

We analyze two approaches to the quantum-classical Liouville (QCL) formalism that differ in the order of two operations: Wigner transformation and projection onto adiabatic electronic states. The analysis is carried out on a two-dimensional linear vibronic model where geometric phase (GP) effects arising from a conical intersection profoundly affect nuclear dynamics. We find that the Wigner-then-Adiabatic (WA) QCL approach captures GP effects, whereas the Adiabatic-then-Wigner (AW) QCL approach does not. Moreover, the Wigner transform in AW-QCL leads to an ill-defined Fourier transform of double-valued functions. The double-valued character of these functions stems from the nontrivial GP of adiabatic electronic states in the presence of a conical intersection. In contrast, WA-QCL avoids this issue by starting with the Wigner transform of single-valued quantities of the full problem. As a consequence, GP effects in WA-QCL can be associated with a dynamical term in the corresponding equation of motion. Since the WA-QCL approach uses solely the adiabatic potentials and non-adiabatic derivative couplings as an input, our results indicate that WA-QCL can capture GP effects in two-state crossing problems using first-principles electronic structure calculations without prior diabatization or introduction of explicit phase factors.

Semiclassical Monte-Carlo approach for modelling non-adiabatic dynamics in extended molecules

Modelling of non-adiabatic dynamics in extended molecular systems and solids is a next frontier of atomistic electronic structure theory. The underlying numerical algorithms should operate only with a few quantities (that can be efficiently obtained from quantum chemistry), provide a controlled approximation (which can be systematically improved) and capture important phenomena such as branching (multiple products), detailed balance and evolution of electronic coherences. Here we propose a new algorithm based on Monte-Carlo sampling of classical trajectories, which satisfies the above requirements and provides a general framework for existing surface hopping methods for non-adiabatic dynamics simulations. In particular, our algorithm can be viewed as a post-processing technique for analysing numerical results obtained from the conventional surface hopping approaches. Presented numerical tests for several model problems demonstrate efficiency and accuracy of the new method.

Many interesting chemical problems like photosynthesis and photovoltaics involve non-adiabatic dynamical phenomena, which are difficult to predict theoretically. Here, the authors develop a new numerical method capable of recovering quantum interferences that are neglected by conventional methods.

Electron-ion coincidence study of photofragmentation of the CdCl(2) molecule

In this work, the photofragmentation subsequent to valence and Cd4d photoionization of cadmium dichloride (CdCl(2)) were studied using He I and synchrotron excitation. The measurements were performed with a photoelectron-photoion coincidence (PEPICO) setup, and the connection between the singly ionized electronic states and cationic fragments was investigated. The valence-ionized states were found to lead to CdCl(2)(+), Cd(+) and CdCl(+). The Cd4d(- 1) states were found to lead only to Cl(+) ions. The observed charge transfer effect between Cd and Cl was concluded to take place due to internal conversion or fluorescence decay to dissociating valence states either directly or through consecutive fragmentation. The fragmentation energetics were investigated with molecular ab initio calculations, and the calculated energies were found to agree with the detected fragment appearances.

Critical appraisal of the fewest switches algorithm for surface hopping

In this paper the authors address the problem of internal consistency in trajectory surface hopping methods, i.e., the requirement that the fraction of trajectories running on each electronic state equals the probabilities computed by the electronic time-dependent Schrodinger equation, after averaging over all trajectories. They derive a formula for the hopping probability in Tully's "fewest switches" spirit that would yield a rigorously consistent treatment. They show the relationship of Tully's widely used surface hopping algorithm with the "exact" prescription that cannot be applied when running each trajectory independently. They also bring out the connection of the consistency problem with the coherent propagation of the electronic wave function and the artifacts caused by coherent Rabi-type oscillations of the state probabilities in weak coupling regimes. A real molecule (azobenzene) and two ad hoc models serve as examples to illustrate the above theoretical arguments. Following a proposal by Truhlar's group [Zhu et al., J. Chem. Phys. 121, 7658 (2004) Zhu et al., J. Chem. Theory Comput. 1, 527 (2005)], they apply a decoherence correction to the state probabilities, in conjunction with Tully's algorithm, and they obtain satisfactory results in terms of internal consistency and of agreement with the outcomes of quantum wave packet calculations.

Can the Nonadiabatic Photodynamics of Aminopyrimidine Be a Model for the Ultrafast Deactivation of Adenine?

The reaction path for the ultrafast deactivation of 6-aminopyrimidine (6AP) has been investigated by means of ab initio surface-hopping dynamics. The electronic vertical excitation spectrum, excited-state S1 minima, and minima on the crossing seam of 6AP resemble well those found for adenine. The deactivation from the S1 to the S0 state takes place at the ultrafast time scale of 400 fs and involves the out-of-plane ring deformation of the C4 atom, a position that is sterically restricted in adenine by the imidazole ring. Mechanical restrictions have been used to simulate in a simple way the role of the imidazole group in adenine. As a result, deactivation via out-of-plane ring deformation of the C2 and N3 atoms are observed in good agreement with predictions for adenine. These dynamics results show that the previously suggested ring puckering deactivation paths really exist at a time scale, which is compatible with experimentally observed life times. The electronic structure of the crossing seam has been shown to have the same nature as those of simple biradicaloid systems, a feature which seems to be valid for any cyclic system.

Quantum-classical dynamics of wave fields

An approach to the quantum-classical mechanics of phase space dependent operators, which has been proposed recently, is remodeled as a formalism for wave fields. Such wave fields obey a system of coupled nonlinear equations that can be written by means of a suitable non-Hamiltonian bracket. As an example, the theory is applied to the relaxation dynamics of the spin-boson model. In the adiabatic limit, a good agreement with calculations performed by the operator approach is obtained. Moreover, the theory proposed in this paper can take nonadiabatic effects into account without resorting to surface-hopping approximations. Hence, the results obtained follow qualitatively those of previous surface-hopping calculations and increase by a factor of (at least) 2, the time length over which nonadiabatic dynamics can be propagated with small statistical errors. Moreover, it is worth to note that the dynamics of quantum-classical wave fields proposed here is a straightforward non-Hamiltonian generalization of the formalism for nonlinear quantum mechanics that Weinberg introduced recently.

Excited-State Charge Transfer at a Conical Intersection: Effects of an Environment

The influence of a polar and polarizable environment on charge transfer processes at a conical intersection (CI) can be described by a diabatic free energy model yielding coupled surfaces as a function of both molecular coordinates and a solvent coordinate. We extend and apply this model for the S1-S0 CI in protonated Schiff bases, representing a model for retinal isomerization (Faraday Discuss. 2004, 127, 395, 2004). A dielectric continuum description of the solvent is combined with a minimal, two-electron-two-orbital electronic structure model according to Bonacić-Koutecký, Koutecký, and Michl (Angew. Chem. 1987, 26, 170), which characterizes the charge translocation effects at the CI. The model predicts that the nonequilibrium solvent state resulting from the S0-->S1 Franck-Condon transition can entail the disappearance of the CI, such that solvent motion is necessary to reach the CI seam. The concerted evolution of the intramolecular coordinates and the solvent coordinate is illustrated by an excited-state minimum energy path.