  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.
