On the use of the quasi-Gaussian entropy theory in systems of polyatomic flexible molecules

In this article we show how the quasi-Gaussian entropy (QGE) theory can be used to treat systems of polyatomic flexible molecules, where the usual sem irigid description is not always appropriate. We describe a completely gene ral derivation of the QGE theory which does not make use of any semirigid a pproximation, and therefore it is very suited for large and flexible molecu les. Using molecular dynamics simulations of flexible molecules in vacuo, w e investigated the ability of the theory to describe intramolecular energy fluctuations and conformational equilibria of purely classical molecules in the ideal gas condition. Results show that the gamma state level of the th eory and its generalization for treating conformational equilibria (multi-g amma state model) provide excellent theoretical models when applied to thre e polyatomic molecules of increasing conformational freedom.