Temperature dependence of water vibrational spectrum: A molecular dynamics simulation study

Vibrational spectroscopy studies show that the bulk water bending band becomes narrower with increasing temperature (Marechal, Y. J. Mol. Struct. 1994, 322, 105). Since this counterintuitive effect is not associated with the quantum nature of nuclear motion a molecular dynamics (MD) simulation is expected to reproduce it even in the classical limit. We have performed a classical MD simulation of the flexible simple point charge (SPC) and extended SPC (SPC/E) water models to determine the temperature dependence of the bulk water vibrational spectrum. The intramolecular water potential proposed by Toukan and Rahman, including a stretch-bend coupling term, was applied. We performed MD simulations at -4 and 80 degreesC to compare the calculated vibrational spectra, in particular, the band associated with the bending mode, with the experiment. The experimentally determined narrowing of the bending band with increasing temperature is not reproducible by MD simulation with the applied force field. However, the results show that this approach successfully reproduces all other experimentally observed spectroscopic properties of bulk water.