Finite temperature anharmonic gas phase vibrational spectroscopy with DFT-based MD simulations*

We will review our works on gas phase vibrational spectroscopy of gas phase molecules and clusters by finite temperature DFT-based molecular dynamics simulations. The anharmonic dynamical spectra have been used in order to make systematic links to the most advanced action spectroscopy experiments.
Our combined theoretical-experimental works on the IR-MPD (Infra Red Multi Photon Dissociation) and IR-PD (Infra Red Pre-Dissociation) action spectroscopies of gas phase molecular ions will be shown, where the importance of conformational dynamics, proton transfers, temperature, potential energy surface and dipolar anharmonicities, entropic effects, high energy conformers, will be emphasized. Our more recent works on the far-IR/THz spectroscopy of gas phase peptides will also be presented in relation with the most advanced IR-UV ion dip experiments probing the 100-800 cm^-1 spectral domain. In this spectral range, DFT-MD anharmonic spectroscopy has proved essential into assigning spectral features to 3D structures, with a robustness not reached by other anharmonic theoretical methods. We will emphasize the importance of large amplitude motions and highly flexible H-Bonds into the 100-800 cm^-1 signatures, and will show which modes are intrinsically anharmonic vs harmonic. This calls for the development of mixed harmonic/anharmonic methods.
Our recent innovative theoretical developments into the calculation of gas phase anharmonic spectra from DFT-MD simulations, going beyond the time-correlation function of dipole moments, will also be presented.