How Much Can Molecular Dynamics Improve on Harmonic Analysis for Vibrational Spectrum Predictions? Insights from Microcanonical Molecular Simulations of Highly 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 paper, we propose a microcanonical molecular simulation scheme for efficient calculations of vibrational spectra of molecular clusters. Within the new scheme, we perform both harmonic analyses and molecular dynamics simulations and predict vibrational spectra of three challenging water clusters: the neutral water dimer (H4O2), the protonated water trimer (H7O3+), and the protonated water tetramer (H9O4+). We find that with the accurate description of quantum nuclear delocalization effects through the constrained nuclear-electronic orbital framework, including vibrational mode coupling effects through molecular dynamics simulations can additionally improve the vibrational spectrum calculations. In contrast, without the quantum nuclear delocalization picture, conventional ab initio molecular dynamics may even lead to less accurate results than harmonic analysis.