Flexible molecular systems studied by quantum chemistry and statistical mechanics

Quantum chemical and statistical mechanical methods are employed to study flexible molecular systems. Using quantum chemical methods it is demonstrated that the preference of the gauche conformation for 1,2-difluoroethane, predicted at the Hartree-Fock level, is due to a larger overlap between the singly occupied orbitals of the CFH2 radicals in the gauche conformation. The intermolecular water potential developed at this laboratory (NEMO) is compared with accurate quantum chemical calculations for water clusters. A generalization of the NEMO method is outlined that also includes intramolecular degrees of freedom. Monte Carlo simulations are performed for water solutions of 1,2-dimethoxyethane (DME)at different temperatures. The problem of obtaining a proper sampling of the phase space in Monte Carlo simulations is examined. A method to create the potential of mean force during the simulation is proposed. The method is exemplified with gas phase simulations of DME and grand canonical simulations of hard spheres between hard walls. Monte Carlo simulations of DME and diglyme at different concentrations in water and formamide solution is performed. Finally, a model for predicting retention indeces in gas-liquid chromatography is outlined.