Adaptive excitation frequencies in simulations of resonance Raman spectra

Within the Born-Oppenheimer approximation, differences of the vibrational energies are often neglected compared to the electronic ones. This leads to the Placzek approximation and the coupled-perturbed methodology, where the transition polarizabilities needed to simulate Raman spectra are calculated at an incident frequency. Within this approximation, the frequency change of the outgoing radiation is neglected, which has a little effect on the simulation of Raman and Raman optical activity spectra in the far from resonance limit. For resonance and pre-resonance, when the incident frequency is close to the energy of an electronic transition, significant error may arise. We analyze this problem and suggest a partial solution through a modification of the polarizability expressions to include a dependence on the vibrational frequencies. For model cobalt complexes, this improved formulation leads to a significant improvement of the spectral patterns.