Vibrational Analysis Beyond the Harmonic Regime From Ab-initio Molecular Dynamics

The characterization of fundamental vibrational levels from the analysis of ab-initio dynamics is discussed. Results from test calculations are compared with those obtained by a second-order perturbative procedure to include anharmonicity exploiting the same ab-initio potential. The influence of the single molecular modes on vibrationally averaged properties calculated on the fly during the dynamics is also analyzed. Formaldehyde, both in the gas phase and in acetonitrile solution, and the vinyl radical in the gas phase have been chosen as test molecules, while the hyperfine coupling constants of the open shell system is computed as an example of molecular properties. Results allow for a quantitative comparison with experimental data.