The MB-pol many-body potential has recently emerged as an accurate molecular model for water simulations from the gas to the condensed phase. In this study, the accuracy of MB-pol is systematically assessed across the three phases of water through extensive comparisons with experimental data and high-level ab initio calculations. Individual many-body contributions to the interaction energies as well as vibrational spectra of water clusters calculated with MB-pol are in excellent agreement with reference data obtained at the coupled cluster level. Several structural, thermodynamic, and dynamical properties of the liquid phase at atmospheric pressure are investigated through classical molecular dynamics simulations as a function of temperature. The structural properties of the liquid phase are in nearly quantitative agreement with X-ray diffraction data available over the temperature range from 268 to 368 K. The analysis of other thermodynamic and dynamical quantities emphasizes the importance of explicitly including nuclear quantum effects in the simulations, especially at low temperature, for a physically correct description of the properties of liquid water. Furthermore, both densities and lattice energies of several ice phases are also correctly reproduced by MB-pol. Following a recent study of DFT models for water, a score is assigned to each computed property, which demonstrates the high and, in many respects, unprecedented accuracy of MB-pol in representing all three phases of water.

The structure of liquid water as a function of temperature is investigated through the modeling of infrared and Raman spectra along with structural order parameters calculated from classical and quantum molecular dynamics simulations with the MB-pol many-body potential energy function. The magnitude of nuclear quantum effects is also monitored by comparing the vibrational spectra obtained from classical and centroid molecular dynamics, both in intensities and peak positions. The observed changes in spectral activities are shown to reflect changes in the underlying structure of the hydrogen-bond network and are found to be particularly sensitive to many-body effects in the representation of the electrostatic interactions. Overall, good agreement is found with the experimental spectra, which provides further evidence for the accuracy of MB-pol in predicting the properties of water.

An unambiguous assignment of the vibrational spectra of ice I remains a matter of debate. This study demonstrates that an accurate representation of many-body interactions between water molecules, combined with an explicit treatment of nuclear quantum effects through many-body molecular dynamics (MB-MD), leads to a unified interpretation of the vibrational spectra of ice I in terms of the structure and dynamics of the underlying hydrogen-bond network. All features of the infrared and Raman spectra in the OH stretching region can be unambiguously assigned by taking into account both the symmetry and the delocalized nature of the lattice vibrations as well as the local electrostatic environment experienced by each water molecule within the crystal. The high level of agreement with experiment raises prospects for predictive MB-MD simulations that, complementing analogous measurements, will provide molecular-level insights into fundamental processes taking place in bulk ice and on ice surfaces under different thermodynamic conditions.

The temperature dependence of the vibrational sum-frequency generation (vSFG) spectra of the air/water interface is investigated using many-body molecular dynamics (MB-MD) simulations performed with the MB-pol potential energy function. The vSFG spectra calculated for different polarization combinations are then analyzed in terms of molecular autocorrelation and cross-correlation contributions. To provide molecular-level insights into interfacial hydrogen-bonding topologies, which give rise to specific spectroscopic features, the vSFG spectra are further investigated by separating contributions associated with water molecules donating zero, one, or two hydrogen bonds to neighboring water molecules. This analysis suggests that the low frequency shoulder of the free OH peak which appears at ∼3600 cm is primarily due to intermolecular couplings between both singly and doubly hydrogen-bonded molecules.

To probe the dynamic nature of the hydrogen bond network in water, linear and nonlinear infrared spectra of dilute HOD in H2O are computed from many-body molecular dynamics simulations with the MB-pol potential, which have been shown to accurately predict the properties of water from the gas to the condensed phase. The effects of various approximations to the many-body expansion of the dipole moment surface on the OD-stretch absorption line shapes are analyzed at different levels of theory. The interplay between effects associated with the variation of the HOD dipole moment and instantaneous nuclear configurations causes qualitative differences in the absorption profiles, which are traced back to how induction contributions are treated within the many-body formalism. Further analysis of the multidimensional infrared spectra demonstrates that the spectral diffusion of the OD stretching frequencies depends explicitly on the level of truncation in the many-body expansion of the dipole moment in the short-time regime that is associated with intact hydrogen-bond dynamics. In contrast, the long-time evolution of spectral diffusion, describing collective rearrangements of the hydrogen-bond network, is effectively independent of the details with which many-body contributions to the dipole moment are represented.