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 pot ential functions representing interatomic interactions is proposed. It is a pplied to derive an intramolecular potential model for the water molecule a ble to reproduce the effects of an electric field on its geometry and dynam ics: to enlarge the HOH angle, to increase slightly the OH bond lengths, to red-shift the stretching vibrational frequencies, and to blue-shift slight ly the bending mode frequency. These effects have been detected experimenta lly for water adsorbed in zeolites and have been confirmed by quantum mecha nical calculations. The electric-field-dependent intramolecular potential m odel for water has been combined with a newly refined intermolecular potent ial for bulk water and with new potentials representing cation-water and al uminosilicate-water interactions in order to simulate, by classical molecul ar dynamics (MD) technique, the behavior of water adsorbed in zeolites. The performances of the model have been checked by a MD simulation of liquid w ater at room temperature, by the structural and vibrational properties of t he water dimer, and by test MD calculations on a hydrated natural zeolite ( natrolite). The results are encouraging, and the simulations will be extend ed to study the behavior of water adsorbed in other zeolites, including dif fusion and some aspects of ion exchange processes. (C) 2000 American Instit ute of Physics. [S0021- 9606(00)50319-5].