An interesting question is whether or not mean-field or “Ehrenfest” mixed quantum-classical dynamics can capture the quantized vibrational features in photoabsorption spectra that result from infrared and Raman-active vibrational transitions. We show that vibrational and electronic absorption spectra can be obtained together within a single Ehrenfest simulation. The electronic transitions show new sidebands that are absent in electronic dynamics simulations with fixed nuclei, and, what’s more, the sideband spacing corresponds to Raman-active vibrational frequencies. To understand this, we present a simple derivation of the time-evolving dipole in the presence of external fields and vibrational motion shows the origin of these features. From this, we demonstrate that mixed quantum-classical Ehrenfest dynamics is capable of producing infrared, Raman, and electronic absorption spectra from a single simulation.
