The role of nonbond and charge flux in hydrogen bond interactions. The effect on structural changes and spectral shifts in water dimer

An analysis of Hessian matrix elements in molecular systems is presented. It is shown that elements that couple nonbonded atoms lead to an exact pairwise decomposition of intermolecular potential energy surfaces. The analysis further shows that potential energy surfaces of dimers in general contain terms that couple inter and intramolecular coordinates. Such terms arise from the "flux" of the internal charge distribution as the monomers distort in response to each other. These fluxes may be represented analytically by allowing atomic parameters such as point charges and van der Waals parameters to be coordinate dependent. Three hydrogen bonded dimers are studied in some detail with special focus on the short-range nonbond repulsions. It is shown that the pairwise hydrogen bond interactions in water-water, water-formaldehyde, and water-formamide are very similar, and that the nonbond flux is effective mainly along the O-H bond of the donor water molecule. The flux and the functional form of the nonbonded interaction in the water dimer are parametrized. Intramolecular structural changes and IR shifts and splittings in this system are shown to be strongly dependent on charge and nonbond flux terms. It is suggested that such flux terms provide a general mechanism for controlling structural and spectral changes in dimers.