PNNL

Abstract

The theory of vibrational spectral analysis from classical trajectories has been available for some time.1-3. It is only recently that, with the availability of powerful computers, it has become practical to apply the method to the ab initio calculation of molecular spectra. These spectra account for anharmonicities and couplings of all the modes. The modern application of the approach combines the methodology of direct ab initio dynamics to generate classical trajectories of the molecular system and of Fourier decomposition of the fluctuations of the velocity autocorrelation function to obtain the density of vibrational states. Infrared and Raman intensities are obtained from Fourier decompositions of the fluctuations of the autocorrelation function of the dipole moment time derivative4 and polarizability time second-derivatives respectively5. In this chapter we review the essential equations of the method and illustrate its application to two simple molecular systems, water H2O and its deuterated analog D2O, and water dimer (H2O)2. The spectra were generated by applying the protocol of quasiclassical direct dynamics with forces obtained on the fly from accurate electron-correlated ab initio levels of theory. In the quasiclassical protocol the initial conditions of atomic velocities (directions and magnitudes) are chosen to be consistent with the level v=0 for all the harmonic normal modes of the molecule. The calculated spectra are found in excellent quantitative agreement with the experimentally observed spectra. A detailed analysis for D2O shows that this choice of initial conditions (quasiclassical with ?=0 for all the modes) offers an accurate account of anharmonicities and mode couplings. The method holds promise for the calculation of very accurate ab initio spectra, including IR and Raman intensities, directly comparable to experimental spectra for increasingly larger molecules.