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.

Despite recent progress, a unified understanding of how ions affect the structure and dynamics of water across different phases remains elusive. Here, we report the development of full-dimensional many-body potential energy functions, called MB-nrg (Many-Body-energy), for molecular simulations of halide ion–water systems from the gas phase to the condensed phase. The MB-nrg potentials are derived entirely from “first-principles” calculations carried out at the F12 explicitly correlated coupled-cluster level including single, double, and perturbative triple excitations, CCSD­(T)-F12, in the complete basis set limit. Building upon the functional form of the MB-pol water potential, the MB-nrg potentials are expressed through the many-body expansion of the total energy in terms of explicit contributions representing one-body, two-body, and three-body interactions, with all higher-order contributions being described by classical induction. The specific focus of this study is on the MB-nrg two-body terms representing the full-dimensional potential energy surfaces (PESs) of the corresponding H2O–X– dimers, with X–= F–, Cl–, Br–, and I–. The accuracy of the MB-nrg PESs is systematically assessed through extensive comparisons with results obtained using both ab initio models and polarizable force fields for energies, structures, and harmonic frequencies of the H2O–X– dimers.

This study presents the extension of the MB-nrg (Many-Body energy) theoretical/computational framework of transferable potential energy functions (PEFs) for molecular simulations of alkali metal ion-water systems. The MB-nrg PEFs are built upon the many-body expansion of the total energy and include the explicit treatment of one-body, two-body, and three-body interactions, with all higher-order contributions described by classical induction. This study focuses on the MB-nrg two-body terms describing the full-dimensional potential energy surfaces of the M(HO) dimers, where M = Li, Na, K, Rb, and Cs. The MB-nrg PEFs are derived entirely from "first principles" calculations carried out at the explicitly correlated coupled-cluster level including single, double, and perturbative triple excitations [CCSD(T)-F12b] for Li and Na and at the CCSD(T) level for K, Rb, and Cs. The accuracy of the MB-nrg PEFs is systematically assessed through an extensive analysis of interaction energies, structures, and harmonic frequencies for all five M(HO) dimers. In all cases, the MB-nrg PEFs are shown to be superior to both polarizable force fields and ab initio models based on density functional theory. As previously demonstrated for halide-water dimers, the MB-nrg PEFs achieve higher accuracy by correctly describing short-range quantum-mechanical effects associated with electron density overlap as well as long-range electrostatic many-body interactions.

New potential energy functions (i-TTM) describing the interactions between halide ions and water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy. Many-body effects are represented through classical polarization within an extended Thole-type model. By construction, the i-TTM potentials are compatible with the flexible and fully ab initio MB-pol potential, which has recently been shown to accurately predict the properties of water from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through extensive comparisons with CCSD(T)-F12, DF-MP2, and DFT data as well as with results obtained with common polarizable force fields for X(-)(H2O)n clusters with X(-) = F(-), Cl(-), Br(-), and I(-), and n = 1-8. By construction, the new i-TTM potentials will enable direct simulations of vibrational spectra of halide-water systems from clusters to bulk and interfaces.

Recent work has shown that the many-body expansion of the interaction energy can be used to develop analytical representations of global potential energy surfaces (PESs) for water. In this study, the role of short- and long-range interactions at different orders is investigated by analyzing water potentials that treat the leading terms of the many-body expansion through implicit (i.e., TTM3-F and TTM4-F PESs) and explicit (i.e., WHBB and MB-pol PESs) representations. It is found that explicit short-range representations of 2-body and 3-body interactions along with a physically correct incorporation of short- and long-range contributions are necessary for an accurate representation of the water interactions from the gas to the condensed phase. Similarly, a complete many-body representation of the dipole moment surface is found to be crucial to reproducing the correct intensities of the infrared spectrum of liquid water.

A microscopic picture of hydrogen-bond structure and dynamics in ion hydration shells remains elusive. Small ion-dihydrate molecular complexes represent ideal systems to investigate the interplay and competition between ion-water and water-water interactions. Here, state-of-the-art quantum dynamics simulations provide evidence for tunneling in hydrogenbond rearrangements in the iodide-dihydrate complex and show that it can be controlled through isotopic substitutions. We find that the iodide ion weakens the neighboring waterwater hydrogen bond, leading to faster water reorientation than in the analogous water trimer. These faster dynamics, which are apparently at odds with the slowdown observed in the first hydration shell of iodide in solution, can be traced back to the presence of a free OH bond in the iodide-dihydrate complex, which effectively triggers the overall structural rearrangements within it. Besides providing indirect support for co-operative hydrogen-

Full-dimensional vibrational spectra are calculated for both X(HO) and X(DO) dimers (X = F, Cl, Br, I) at the quantum-mechanical level. The calculations are carried out on two sets of recently developed potential energy functions (PEFs), namely, Thole-type model energy (TTM-nrg) and many-body energy (MB-nrg), using the symmetry-adapted Lanczos algorithm with a product basis set including all six vibrational coordinates. Although both TTM-nrg and MB-nrg PEFs are derived from coupled-cluster single double triple-F12 data obtained in the complete basis set limit, they differ in how many-body effects are represented at short range. Specifically, while both models describe long-range interactions through the combination of two-body dispersion and many-body classical electrostatics, the relatively simple Born-Mayer functions employed in the TTM-nrg PEFs to represent short-range interactions are replaced in the MB-nrg PEFs by permutationally invariant polynomials to achieve chemical accuracy. For all dimers, the MB-nrg vibrational spectra are in close agreement with the available experimental data, correctly reproducing anharmonic and nuclear quantum effects. In contrast, the vibrational frequencies calculated with the TTM-nrg PEFs exhibit significant deviations from the experimental values. The comparison between the TTM-nrg and MB-nrg results thus reinforces the notion that an accurate representation of both short-range interactions associated with electron density overlap and long-range many-body electrostatic interactions is necessary for a correct description of hydration phenomena at the molecular level.

Replica exchange molecular dynamics simulations and vibrational spectroscopy calculations are performed using halide−water many-body potential energy functions to provide a bottom-up analysis of the structures, energetics, and hydrogen-bonding arrangements in X − (H 2 O) n (n = 3−6) clusters, with X = F, Cl, Br, and I. Independently of the cluster size, it is found that all four halides prefer surfacetype structures in which they occupy one of the vertices in the underlying three-dimensional hydrogen-bond networks. For fluoride−water clusters, this is in contrast to previous reports suggesting that fluoride prefers interior-type arrangements, where the ion is fully hydrated. These differences can be ascribed to the variability in how various molecular models are capable of reproducing the subtle interplay between halide−water and water−water interactions. Our results thus emphasize the importance of a correct representation of individual many-body contributions to the molecular interactions for a quantitative description of halide ion hydration.