Combined time correlation function and instantaneous normal mode investigation of liquid-state vibrational spectroscopy

Our work investigates the instantaneous normal mode (INM) and time correlation function (TCF) of several different liquids and compares the results with the experimental infrared (IR) and Raman spectra. Our work demonstrates that INM and TCF methods can be used in a complementary fashion in describing liquid state vibrational spectroscopy. INM derived spectra often lead to intermolecular spectra that are in agreement with corresponding TCF results, suggesting that the inter-molecular dynamics can be interpreted as oscillations. TCF derived spectra, while formally exact in the low frequency regime (homega much less than kT), suffer severely from the need for quantum (detailed balance) correction at higher frequencies. Our approach is to compare TCF spectra with experiment to establish that our MD methods can reliably describe the system of interest, and to employ INM methods to analyze the molecular and dynamical basis for the observed spectroscopy. We have been able to elucidate, on a molecularly detailed basis, a number of condensed phase line shapes, most notably the origin of the broad and intense, high frequency intramolecular O-H stretching absorption in liquid water. This approach has proven successful in systems as diverse as liquid CS2 and H2O at a number of state points.