Vibrational Analysis of an Ice Ih Model from 0 to 4000 cm–1 Using the Ab Initio WHBB Potential Energy Surface

Abstract

We present an analysis of the vibrational modes of a model of hexagonal ice, ice Ih, comprised of 192 monomers with a core region of 105 monomers, using the ab initio WHBB potential energy surface [Wang, Y.; Shepler, B.; Braams, B.; Bowman, J. M. J. Chem. Phys. 2011, 134, 094509]. A standard normal-mode analysis and a local-monomer normal-mode analysis of 105 core monomers are performed to obtain harmonic frequencies and state densities of the "pseudo-translation" (0-400 cm(-1)), "libration" (500-1100 cm(-1)), monomer bend fundamental (~1600 cm), and O-H stretch (~3000-3700 cm(-1)) bands. In addition, the coupled local-monomer model is used to obtain the vibrational density of states in the bend fundamental and O-H stretch regions. The harmonic and local-monomer vibrational density of states obtained from core monomers are in good agreement with those of inelastic neutron scattering spectra, especially the latter, which accounts for anharmonic coupling of monomer modes. Full deuteration is also considered, and the vibrational density of states is again compared to experiment, where good agreement is found.