UC San Diego

Due to the sensitivity of molecular vibrational frequencies and intensities on the surrounding environment, vibrational spectroscopies in principle enable the study of solvation structure and dynamics. Connecting the observed spectral features to a molecular-level picture is, however, often non-trivial. While computer simulations of molecular dynamics represent a potentially powerful tool for developing this molecular-level understanding, the accurate simulation of vibrational spectroscopies in condensed phases poses significant challenges due to the sensitivity of the spectra on both the underlying molecular interactions and the difficulty of obtaining a (statistically meaningful) treatment of the quantum dynamics. In this work, we begin by assessing the ability of different molecular models to reproduce thousands of reference two- and three-body interaction energies calculated at the current "gold standard" level of electronic structure theory, CCSD(T). As described in Chapter 2, these results led us to develop a potential energy surface, named MB-pol, that was fitted exclusively to large datasets of CCSD(T) many-body interaction energies. Crucially, MB-pol was designed to be computationally tractable for condensed phase simulations without sacrificing accuracy. MB-pol reproduces experimental measurements of small cluster properties, as well as thermodynamic and dynamical properties of bulk water at ambient conditions, without containing any empirically derived parameters (Chapter 3). However, unlike the electronic structure calculations to which it is fitted, the MB- pol PES contains no explicit knowledge of the electron distribution, which is required for the calculation of vibrational spectra. To this end, in Chapter 4 we demonstrate that the many-body expansions of the dipole and polarizability also converge for water. Based on this finding, in Chapter 5 we introduce many-body models for the dipole moment and polarizability of water, allowing us to rigorously model IR and Raman spectra from "first principles," through the respective (approximate) quantum time correlation functions. In Chapter 6, we disentangle the contributions of the potential energy and dipole moment surfaces to the IR activity of liquid water. Finally, we conclude in Chapter 7 by reflecting on possible future applications, including the application of the MB-MD approach to the calculation of nonlinear vibrational spectra