Comparison of path integral molecular dynamics methods for the infrared absorption spectrum of liquid water.

The ring polymer molecular dynamics (RPMD) and partially adiabatic centroid molecular dynamics (PA-CMD) methods are compared and contrasted in an application to the infrared absorption spectrum of a recently parametrized flexible, polarizable, Thole-type potential energy model for liquid water. Both methods predict very similar spectra in the low-frequency librational and intramolecular bending region at wavenumbers below 2500 cm(-1). However, the RPMD spectrum is contaminated in the high-frequency O-H stretching region by contributions from the internal vibrational modes of the ring polymer. This problem is avoided in the PA-CMD method, which adjusts the elements of the Parrinello-Rahman mass matrix so as to shift the frequencies of these vibrational modes beyond the spectral range of interest. PA-CMD does not require any more computational effort than RPMD and it is clearly the better of the two methods for simulating vibrational spectra.