An effective bath state approach to model infrared spectroscopy and intramolecular dynamics in complex molecules

When a molecule contains more than a few atoms, its full-dimensional dynamics becomes untractable, especially when introducing temperature effects. In such cases, it can be interesting to focus only on a few degrees of freedom and model the rest of the molecule as a finite-dimensional bath. In this prospect, we extend the effective bath state (EBS) method that we had first developed and benchmarked in Attal et al. [J. Chem. Phys. 160, 044107 (2024)] to describe the spectroscopy and intramolecular dynamics of complex isolated molecules. The EBS method is a system-bath approach based on the coarse-graining of the bath into a reduced set of effective energy states. It allows for a significant reduction of the bath dimension and makes finite-temperature calculations more accessible. In order to treat a realistic molecule, the method is extended to include polynomial couplings in the bath coordinates. The ability of the method to model temperature-resolved infrared spectra and to follow population transfers between the vibrational modes of the molecule is first tested on a ten-mode model system. The extended method is then applied to the realistic case of phenylacetylene.