On-the-fly <i>ab initio</i> semiclassical evaluation of vibronic spectra at finite temperature [1]

To compute vibrationally resolved electronic spectra at zero temperature, we recently implemented the on-the-fly ab initio extended thawed Gaussian approximation, which accounts for anharmonicity, mode-mode coupling, and Herzberg-Teller effects. Here, we generalize this method to spectra at non-zero temperature. As in thermo-field dynamics, we transform von Neumann's evolution of the coherence component of the density matrix to the Schrödinger evolution of a wavefunction in an augmented space with twice as many degrees of freedom. Due to efficiency of the thawed Gaussian approximation, this increase in dimensionality results in nearly no additional computational cost: compared to the zero-temperature version, the finite-temperature method requires no additional ab initio electronic structure calculations. The new approach allows for a clear distinction among finite-temperature, anharmonicity, and Herzberg--Teller effects on spectra. We show, on a model Morse system, the advantages of the finite-temperature thawed Gaussian approximation over the popular global harmonic methods and apply it to evaluate the symmetry-forbidden absorption spectrum of benzene, where all of the aforementioned effects contribute.
[1] J. Chem. Phys. 153, 024105 (2020). Editor's pick and featured article.