Efficient on-the-fly ab initio semiclassical method for computing time-resolved nonadiabatic electronic spectra with surface hopping or Ehrenfest dynamics

Abstract

Articles you may be interested inEvaluation of the importance of spin-orbit couplings in the nonadiabatic quantum dynamics with quantum fidelity and with its efficient "on-the-fly" ab initio semiclassical approximation J. Chem. Phys. 137, 22A516 (2012) We derive a somewhat crude, yet very efficient semiclassical approximation for computing nonadiabatic spectra. The resulting method, which is a generalization of the multiple-surface dephasing representation, includes quantum effects through interference of mixed quantum-classical trajectories and through quantum treatment of the collective electronic degree of freedom. The method requires very little computational effort beyond the fewest-switches surface hopping or Ehrenfest locally mean-field dynamics and is very easy to implement. The proposed approximation is tested by computing the absorption and time-resolved stimulated emission spectra of pyrazine using the fourdimensional three-surface model which allows for comparison with the numerically exact quantum spectra. As expected, the multiple-surface dephasing representation is not suitable for high-resolution linear spectra, yet it seems to capture all the important features of pump-probe spectra. Finally, the method is combined with on-the-fly ab initio evaluation of the electronic structure (i.e., energies, forces, electric-dipole, and nonadiabatic couplings) in order to compute fully dimensional nonadiabatic spectra of pyrazine without approximations inherent to analytical, including vibronic-coupling models. The Appendix provides derivations of perturbative expressions for linear and pump-probe spectra of arbitrary mixed states and for arbitrary laser pulse shapes. © 2014 AIP Publishing LLC.

Nonadiabatic semiclassical dynamics in the mixed quantum-classical initial value representation

“…6,7 Over the past two decades, several methods for the simulation of nonadiabatic processes have been developed including exact quantum time-propagation, [8][9][10] the symmetrical quasi-classical windowing method, 11 mixed quantum-classical Liouville methods, [12][13][14] and surface hopping. [15][16][17][18][19][20][21][22] In addition, approximate path-integral based methods such as ring polymer molecular dynamics [23][24][25][26][27] and centroid molecular dynamics 28 have also been extended to nonadiabatic systems. [29][30][31][32][33][34][35][36][37][38] However, while exact quantum methods are limited to a small number of degrees of freedom (dofs), the more approximate methods fail to capture nuclear quantum coherence effects.…”

“…(19) is invariant to the transformation in Eq. (21). Since the MInt algorithm is Hamiltonian evolution discretized by a symplectic method, there exists a modified HamiltonianȞ whose energy the algorithm conserves exponentially well over exponentially long time intervals.…”

Spectral lineshapes in nonlinear electronic spectroscopy

“…the spectral diffusion), and the fluctuations of the respective oscillator strengths during the underlying ultrafast dynamics. [1][2][3][4] Another essential requirement for modeling nonlinear spectra of photoexcited molecules is the incorporation of higher lying ES. This is desirable for two reasons: first, absorptions to these states (excited state absorptions, ESA) dominate the spectra due to the high spectral density in the visible (Vis) and ultraviolet (UV) region; [5][6][7] second, ESA, together with the stimulated emission (SE) represent characteristic signatures of each photophysical or photochemical decay channel, and hence, their proper description will allow us to recognize electronic structural changes during the dynamics, as well as to disentangle dynamics occurring via competing channels.…”

Several Semiclassical Approaches to Time-resolved Spectroscopy

“…If weak, these processes lead only to the broadening of the spectra, but, if strong, they can completely change the spectral line shapes. To address this issue, we have generalized the dephasing representation to the setting of coupled electronic states and obtained the multiplesurface dephasing representation [42] that can capture the major consequences of the nonadiabatic or spin-orbit couplings on ultrafast electronic spectra. This method can and has been combined with an on-the-fly ab initio evaluation of energies, forces, and nonadiabatic couplings.…”