Electric-field-dependent empirical potentials for molecules and crystals: a first application to flexible water molecule adsorbed in zeolites

A general method to include electric-field-dependent terms in empirical potential functions representing interatomic interactions is proposed. It is applied to derive an intramolecular potential model for the water molecule able to reproduce the effects of an electric field on its geometry and dynamics: to enlarge the HOH angle, to increase slightly the OH bond lengths, to red-shift the stretching vibrational frequencies, and to blue-shift slightly the bending mode frequency. These effects have been detected experimentally for water adsorbed in zeolites and have been confirmed by quantum mechanical calculations. The electric-field-dependent intramolecular potential model for water has been combined with a newly refined intermolecular potential for bulk water and with new potentials representing cation–water and aluminosilicate–water interactions in order to simulate, by classical molecular dynamics (MD) technique, the behavior of water adsorbed in zeolites. The performances of the model have been checked by a MD simulation of liquid water at room temperature, by the structural and vibrational properties of the water dimer, and by test MD calculations on a hydrated natural zeolite (natrolite). The results are encouraging, and the simulations will be extended to study the behavior of water adsorbed in other zeolites, including diffusion and some aspects of ion exchange processes.