A semiclassical investigation of glycine vibrational frequencies

We present a semiclassical study of vibrational energy levels of glycine. It is based on a multiple coherent approach integrated with monodromy matrix regularization for chaotic dynamics. All four lowest-energy glycine conformers are investigated by means of single-trajectory semiclassical spectra obtained upon classical evolution of on-the-fly trajectories with harmonic zero-point energy. For the most stable conformer I, additional on-the-fly trajectories are run for each vibrational mode with energy equal to the first harmonic excitation, and refined vibrational energies are eventually obtained. An analysis of trajectories evolved up to 50000 atomic time units demonstrates that, in this time span, conformer II and III can be considered as isolated species, while conformer I and conformer IV show a pretty facile interconversion. However, this I-IV interconversion does not seem to have a substantial influence on vibrational energy levels, thus validating previous studies based on the assumption of isolated conformers.