Deoxyguanosine and Ac-Phe-Met-NH2 vibrational spectra: a comparison between ab-initio and force field molecular dynamics

The theory of semiclassical dynamics has recently been demonstrated to be an important and powerful method in the field of vibrational spectroscopy. In particular, it has been successfully applied not only to the calculation of small and medium sized isolated molecules spectra but also to the study of complex systems, like water clusters and the Zundel cation. Furthermore, semiclassical dynamics may be efficiently associated to ab-initio molecular dynamics (AIMD) when the focus is on high dimensional systems. This combination showed very good accuracy, but unfortunately it presents high computational costs. In contrast, empiric Force Fields (FF) are known to be fast and computationally cheap. Here we present the vibrational power spectra of two biological systems: the Deoxyguanosine and the Ac-Phe-Met-NH2 dipeptide in their very interesting high frequency region. By employing different approaches (i.e. at harmonic, semiclassical and classical level), we discuss the validity of the Amber94 force field against the performance of DFT-based AIMD. We find that the semiclassical method associated to the FF potential gives the worst results, while better estimates are obtainable through classical formulations and via harmonic frequencies calculations. In particular, Amber94 is accurate for the normal modes associated to simple molecular motions, while it is poor for more complex normal modes. Conversely, AIMD always leads to accurate results. In light of these findings, we conclude that the Amber94 force field should be revised to permit a reliable semiclassical vibrational analysis, at least for in vacuo simulations.