Applicability of the thawed Gaussian wavepacket dynamics to the calculation of vibronic spectra of molecules with double-well potential energy surfaces

TL;DR

This paper assesses the single-Hessian thawed Gaussian wavepacket method for simulating vibronic spectra of molecules with double-well potentials, showing it outperforms harmonic approximations in certain anharmonic cases.

Contribution

It demonstrates the applicability and limitations of a semiclassical wavepacket approach for complex molecular spectra involving double-well potentials, with an improved autocorrelation algorithm.

Findings

More accurate spectra than harmonic approximation.

Effective when wavepackets are at the barrier top or have enough energy to cross barriers.

Requires only a single classical trajectory for simulations.

Abstract

Simulating vibrationally resolved electronic spectra of anharmonic systems, especially those involving double-well potential energy surfaces, often requires expensive quantum dynamics methods. Here, we explore the applicability and limitations of the recently proposed single-Hessian thawed Gaussian approximation for the simulation of spectra of systems with double-well potentials, including 1,2,4,5-tetrafluorobenzene, ammonia, phosphine, and arsine. This semiclassical wavepacket approach is shown to be more robust and to provide more accurate spectra than the conventional harmonic approximation. Specifically, we identify two cases in which the Gaussian wavepacket method is especially useful due to the breakdown of the harmonic approximation: (i) when the nuclear wavepacket is initially at the top of the potential barrier but delocalized over both wells, e.g., along a low-frequency mode, and (ii) when the wavepacket has enough energy to classically go over the low potential energy barrier connecting the two wells. The method is efficient and requires only a single classical ab initio molecular dynamics trajectory, in addition to the data required to compute the harmonic spectra. We also present an improved algorithm for computing the wavepacket autocorrelation function, which guarantees that the evaluated correlation function is continuous for arbitrary size of the time step.