THE OH STRETCHING FREQUENCY IN LIQUID WATER SIMULATIONS - THE CLASSICAL ERROR

OH stretching frequencies of HDO molecules in liquid water have been calculated by molecular dynamics simulation and compared to quantum-corrected OH stretching frequencies. In the MD simulation the MCY intermolecular water-water potential was used together with an experimental intramolecular free water potential. The frequencies calculated classically by Fourier transformation of the velocity autocorrelation function are found to be almost-equal-to 300 cm-1 too high compared with experiment. Quantum corrections show that the classical error contribution to this discrepancy is almost-equal-to 140 cm-1. To reach full agreement with experiment also the potential model needs to be improved. It is suggested that in constructing flexible water potentials the goal should not be set for an MD-derived OH frequency in absolute agreement with experiment (at 3400 cm-1) but instead some 200 cm-1 higher.