How Much Can Molecular Dynamics Improve over Harmonic Analysis on Vibrational Spectrum Predictions? Insights from Microcanonical Molecular Simulations of Anharmonic Water Clusters

Vibrational spectroscopy is widely used to gain insights into structural and dynamic properties of chemical, biological and material systems. Thus, an efficient and accurate method to simulate vibrational spectra is desired. In this Letter, we propose a microcanonical molecular simulation scheme for efficient calculations of vibrational spectra. Within the new scheme, we perform constrained nuclear-electronic orbital molecular dynamics simulations and accurately predict vibrational spectra of three challenging water clusters: neutral water dimer (H4O2), protonated water trimer (H7O3+) and protonated water tetramer (H9O4+). We find that in addition to nuclear quantum effects, vibrational mode coupling effects are also crucial for the accurate description of the vibrational motions of these highly anharmonic hydrogen bonded systems, which accounts for the large discrepancy between the vibrational frequencies obtained from molecular simulations and harmonic analyses.