Monte Carlo simulation of solvent effects on vibrational and electronic spectra

Solvent effects on vibrational states are treated in the Born–Oppenheimer approximation. A Monte Carlo procedure is devised for simulating a system in which the vibrations are treated as quantum degrees of freedom and all other degrees of freedom are treated classically. The key to this method is the use of a restructured perturbation theory for solving the vibrational Schrödinger equation at each of the solvent configurations sampled. This method also is used to compute Franck–Condon factors. The methods developed are applied to two different problems. First, solvent effects on vibrational transitions in the ground electronic state are considered. It is found that the frequency shift from the gas phase frequency is to the red at low solvent densities, but to blue at high solvent densities. Secondly, the heterogeneous electronic absorption and emission line shapes are simulated in a variety of model solute–solvent systems and the results are discussed in terms of a simple theoretical model.