PNNL

Abstract

We illustrate the use of quasi-classical direct molecular dynamics on accurate electron-correlated ab initio potential energy surfaces to extract infrared and Raman intensities in addition to the vibrational density of states of a molecular system. We applied the method to deuterated water D2O using the ab initio MP2 level of electronic structure theory. The initial conditions of kinetic and potential energies of the trajectories were chosen to be consistent with the level v=0 of the harmonic normal modes. We obtained the density of states and the IR and Raman intensities by means of the Fourier transforms of the autocorrelation functions of the velocities, dipole moment time-derivatives, and polarizability tensor time-second-derivatives respectively. The calculated spectrum is in excellent quantitative agreement with the experimentally observed spectrum. The method holds promise to obtain very accurate simulated spectra for small molecules, including IR and Raman intensities. Extensions to the characterization of vibrational effects on the non-linear optical properties of molecules are suggested.