On-the-fly ab initio Hagedorn wavepacket dynamics: single vibronic level fluorescence spectra of difluorocarbene

Hagedorn wavepackets [1][2] have been used with local harmonic approximation to partially capture the anharmonic effects on single vibronic level (SVL) spectra in model potentials [3]. To make the Hagedorn approach practical for realistic anharmonic polyatomic molecules, 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 [4] 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.

[1] G. A. Hagedorn, Annals of Physics, 269(1), 77–104 (1998)

[2] C. Lasser, C. Lubich, Acta Numerica, 29, 229–401 (2020)

[3] Z. T. Zhang, M. Visegrádi, J. J. L. Vaníček, arXiv:2408.11991 [physics.chem-ph] (2024)

[4] Z. T. Zhang, J. J. L. Vaníček, J. Chem. Phys. 161, 111101 (2024)