Ab Initio Wavelength-Dependent Raman Spectra: Placzek Approximation and Beyond.

We analyze how to obtain non-resonant and resonant Raman spectra within the Placzek as well as the Albrecht approximation. Both approximations are derived from the matrix element for light scattering by application of the Kramers, Heisenberg, and Dirac formula. It is shown that the Placzek expression results from a semi-classical approximation of the combined electronic and vibrational transition energies. Molecular hydrogen, water, and butadiene are studied as test cases. It turns out that the Placzek approximation agrees qualitatively with the more accurate Albrecht formulation even in the resonant regime for the excitations of single vibrational quanta. However, multiple vibrational excitations are absent in Placzek but can be of similar intensities to single excitations under resonance conditions. The Albrecht approximation takes multiple vibrational excitations into account, and the resulting simulated spectra also exhibit good agreement with experimental Raman spectra in the resonance region.