On-the-fly ab initio semiclassical evaluation of time-resolved electronic spectra

Abstract

We present a methodology for computing vibrationally and time-resolved pump-probe spectra, which takes into account all vibrational degrees of freedom and is based on the combination of the thawed Gaussian approximation with on-the-fly ab initio evaluation of the electronic structure. The method is applied to the phenyl radical and compared with two more approximate approaches based on the global harmonic approximation-the global harmonic method expands both the ground-and excitedstate potential energy surfaces to the second order about the corresponding minima, while the combined global harmonic/on-the-fly method retains the on-the-fly scheme for the excited-state wavepacket propagation. We also compare the spectra by considering their means and widths, and show analytically how these measures are related to the properties of the semiclassical wavepacket. We find that the combined approach is better than the global harmonic one in describing the vibrational structure, while the global harmonic approximation estimates better the overall means and widths of the spectra due to a partial cancellation of errors.Although the full-dimensional onthe-fly ab initio result seems to reflect the dynamics of only one mode, we show, by performing exact quantum calculations, that this simple structure cannot be recovered using a one-dimensional model. Yet, the agreement between the quantum and semiclassical spectra in this simple, but anharmonic model lends additional support for the full-dimensional ab initio thawed Gaussian calculation of the phenyl radical spectra. We conclude that the thawed Gaussian approximation provides a viable alternative to the expensive or unfeasible exact quantum calculations in cases, where low-dimensional models are not sufficiently accurate to represent the full system.

Ab Initio Semiclassical Evaluation of Vibrationally Resolved Electronic Spectra With Thawed Gaussians

“…For wavepackets generated this way, the on-the-fly and combined methods are equivalent in the linear spectra calculations. In contrast, in the simulations of time-resolved spectra, the nuclear dynamics is performed in both electronic states; as a result, the global harmonic, combined, and on-the-fly approaches all give different results (Begušić et al, 2018b).…”

“…Within the Condon and ultrashort pulse approximations, the ground-state bleach contribution to the spectrum is much simpler to evaluate because it is equal to a scaled linear absorption spectrum discussed in Section V A. Figure 11 shows the time-resolved stimulated emission spectrum of phenyl radical, computed with the on-the-fly ab initio TGA in Begušić et al (2018b). Because the Herzberg-Teller contribution to the absorption spectrum of phenyl radical is negligible (Begušić et al, 2018a;Patoz et al, 2018), the Condon approximation was used for the time-resolved spectrum.…”

“…For a thawed Gaussian wavepacket ψ(τ ) and within the local harmonic approximation, the mean and width of the spectra can be evaluated analytically (Begušić et al, 2018b) as…”

On-the-Fly Symmetrical Quasi-Classical Dynamics with Meyer–Miller Mapping Hamiltonian for the Treatment of Nonadiabatic Dynamics at Conical Intersections

“…[1][2][3] Various theoretical approaches were developed to simulate the nonadiabatic dynamics, [1][2][3][4][5][6][7][8][9][10] which for example include the full quantum dynamics 2,[11][12][13][14][15] and different versions of mixed quantum-classical/semi-classical dynamics. [1][2][5][6][8][9][10][16][17][18][19][20][21][22][23][24][25][26] The complex potential energy functions of the polyatomic molecules with many DoFs impose some additional challenging in the simulation of the nonadiabatic dynamics. To solve such problems, the combination of on-the-fly simulation with various dynamics methods becomes popular in the simulation of the real-time nonadiabatic dynamics of polyatomic systems at all atomic level.…”

On-the-Fly ab initio Semiclassical Evaluation of Electronic Coherences in Polyatomic Molecules Reveals a Simple Mechanism of Decoherence

“…Here, we use the thawed Gaussian approximation (TGA) [37][38][39], one of the simplest semiclassical approaches for molecular dynamics, to evaluate the influence of nuclear motion on the ultrafast electronic dynamics and to find a simple, yet detailed mechanism of decoherence, which is not available in basis set methods such as MCTDH or VMCG. Within the TGA, the nuclear wave function is described by a single Gaussian wave packet whose center follows Hamilton's equations of motion and whose timedependent width and phase are propagated using the local harmonic approximation of the PES.…”