Evaluation of vibrational partition functions for polyatomic systems: quantum ersus classical methods for H2O and Ar···CN

Abstract

The vibrational partition function of H₂O and Ar···CN systems is calculated within the framework of quantum and classical statistical mechanics. The phase space integral arising in the classical picture is solved adopting an efficient Monte Carlo technique. The temperature dependence of the partition function for the two molecules is exploited with a view to study the range of applicability of classical statistical mechanics. For the case of Ar···CN van der Waals complex the role played by freezing the CN bond is also analyzed.