On the accuracy of the MB-pol many-body potential for water: Interaction energies, vibrational frequencies, and classical thermodynamic and dynamical properties from clusters to liquid water and ice

Abstract

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.

Nuclear quantum effects enter the mainstream

“…However, it is important to note that even in these cases the effect on different properties of the system vary dramatically. For example, in water recent advances have allowed the evaluation of NQEs using potential energy surfaces fit to high level ab initio calculations [102] and generated on-the-fly using density functional theory (DFT) [46]. These studies, which give excellent agreement with the structural and dynamical properties of water when NQEs are included, suggest that the NQEs on the diffusion constant and O-O radial distribution function of liquid water are small.…”

Mass density fluctuations in quantum and classical descriptions of liquid water

“…57 MB-pol model was also found to produce the experimental pattern of changing isothermal compressibility with temperature. 57 As an additional self-consistent check to the estimates of the isothermal compressibility that were provided above, we examine the impact of both the thermostat and barostat frequency on our results.…”

Grand Canonical adaptive resolution simulation for molecules with electrons: A theoretical framework based on physical consistency

“…For other systems there may be the need of specific modifications of the general set up. Thus, having in mind water, in order to have some structural consistency between the QM molecules and the classical atomistic molecules, we will assume that the classical atomistic molecular model is not rigid but has flexible bonds and angles, as for example in [59]. Moreover, in first approximation, I will consider the MM region and the hybrid MMcoarse-grained region as a transition region/artificial filter between the QM region and the large coarse-grained region/reservoir.…”