On-the-Fly Ab Initio Hagedorn Wavepacket Dynamics: Single Vibronic Level Fluorescence Spectra of Difluorocarbene

TL;DR

This paper introduces an on-the-fly ab initio Hagedorn wavepacket method for calculating single vibronic level fluorescence spectra of anharmonic molecules, demonstrated on difluorocarbene, showing good agreement with experiments.

Contribution

It combines local harmonic Hagedorn wavepacket dynamics with on-the-fly ab initio calculations to efficiently compute spectra of anharmonic molecules without fitting vibrational wavefunctions.

Findings

Accurately reproduces experimental spectra for difluorocarbene.

Global harmonic models are inadequate for CF2.

Method efficiently computes spectra from a single trajectory.

Abstract

Hagedorn wavepackets have been used to compute single vibronic level (SVL) spectra efficiently in model harmonic potentials. To make the Hagedorn approach practical for realistic polyatomic molecules with anharmonicity, here we combine local harmonic Hagedorn wavepacket dynamics with on-the-fly ab initio dynamics. We then test this method by computing the SVL fluorescence spectra of difluorocarbene, a small, floppy molecule with a very anharmonic potential energy surface. Our time-dependent approach obtains the emission spectra of all initial vibrational levels from a single anharmonic semiclassical wavepacket trajectory without the need to fit individual anharmonic vibrational wavefunctions and to calculate the Franck-Condon factors for all vibronic transitions. We show that, whereas global harmonic models are inadequate for CF2, the spectra computed with the on-the-fly local harmonic Hagedorn wavepacket dynamics agree well with experimental data, especially for low initial excitations.