SIMULATION OF BAND WIDTHS IN LIQUID WATER SPECTRA - THE BREAKDOWN OF THE FROZEN-FIELD APPROXIMATION

Band shapes of liquid water OH vibrational spectra oblained from molecular dynamics (MD) simulation and from a quantum-mechanical method are investigated. The so-called "frozen-field approximation" applied to the calculation of quantum-mechanical high-frequency vibrational spectra is critically examined. It is demonstrated that the band width of the OH stretching spectrum is seriously overestimated through the neglect of the dynamics of the environment in the frozen-field approximation. We show that the proper inclusion of the dynamics in this quantum-mechanical method leads not only to a correct absolute frequency for the model potential used, but also to the correct description of the band width. The basic steps in this method are: (1) an MD simulation yielding an ensemble of liquid water configurations, (2) a quantum-mechanical uncoupled local-mode calculation of the OH frequency for each molecule, using model potentials for the inter- and intra-molecular interactions, (3) inclusion of the influence from the dynamics of the surroundings by filtering out rapid frequency fluctuations. The remaining discrepancy between experimental and computed OH spectra is attributed to shortcomings in the potential model used.