Molecular dynamics analysis of charge fluctuations associated with far-infrared absorption in water

Abstract

Far-infrared absorption spectroscopy gives information on the charge fluctuations associated with hydrogen-bond dynamics in water. Here, we examine these fluctuations by calculating spectra from molecular dynamics simulations and comparing with experiment. Permanent and induced dipoles are included in the calculation of the system dipole moment. The induced dipoles are calculated on each atom using an iterative method that leads to self-consistency of the induced dipole contribution to the local field. The experimental far-infrared spectrum has two prominent bands, at ∼ 600 cm^-1 and ∼ 200 cm^-1, due to hydrogen-bond libration and stretching, respectively. The librational band is found to arise from fluctuations of the positions of the permanent charges in the simulation, whereas the stretching band originates from induced dipole fluctuations. The use of the self-consistent method enhances the intensity of the stretching peak relative to a non-iterative method, producing improved agreement with experiment. The frequency of the librational band is influenced significantly by long-range electrostatic interactions in the potential function. In contrast, the induced dipole fluctuations are relatively insensitive to the long-range effects; the effective isotropy of the molecular polarizability tensor is found to lead to a decoupling of the induced dipole fluctuations from the dynamical intermolecular orientational correlations of the permanent dipoles.