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

journal of chemical theory and computation

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