Mixed quantum/semiclassical wave-packet dynamical method for condensed-phase molecular spectroscopy signals.

We report the successful application of a recently developed mixed quantum/semiclassical wave-packet dynamical theory to the calculation of a spectroscopic signal, the linear absorption spectrum of a realistic small-molecule chromophore in a cryogenic environment. This variational fixed vibrational basis/Gaussian bath (FVB/GB) theory avails itself of an assumed time scale separation between a few, mostly intramolecular, high-frequency nuclear motions and a larger number of slower degrees of freedom primarily associated with an extended host medium. The more rapid, large-amplitude system dynamics is treated with conventional basis-set methods, while the slower time-evolution of the weakly coupled bath is subject to a semiclassical, thawed Gaussian trial form that honors the overall vibrational ground state, and hence the initial state prepared by its Franck-Condon transfer to an excited electronic state. We test this general approach by applying it to a small, symmetric iodine-krypton cluster suggestive of molecular iodine embedded in a low-temperature matrix. Because of the relative simplicity of this model complex, we are able to compare the absorption spectrum calculated via FVB/GB dynamics using Heller's time-dependent formula with one obtained from rigorously calculated eigenenergies and Franck-Condon factors. The FVB/GB treatment proves to be accurate at approximately 15-cm-1 resolution, despite the presence of several thousand spectral lines and a sequence of various-order system-bath resonances culminating at the highest absorption frequencies in an inversion of the relative system and bath time scales.