Article pubs.acs.org/JCTC
Toward Chemical Accuracy in the Description of Ion−Water Interactions through Many-Body Representations. I. Halide−Water Dimer Potential Energy Surfaces Pushp Bajaj,† Andreas W. Götz,‡ and Francesco Paesani*,† †
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States San Diego Supercomputer Center, University of California, San Diego, La Jolla, California 92093, United States
‡
S Supporting Information *
ABSTRACT: 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-Bodyenergy), 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 MBnrg 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.
1. INTRODUCTION Ionic solutions are ubiquitous in nature and often mediate fundamental chemical, biological, and environmental processes. For example, ion-containing aqueous aerosols play a key role in the chemistry that takes place in the atmosphere, having immediate effects on air quality and long-term effects on climate and the environment.1−4 The presence of specific ions alters significantly the structural, thermodynamics, and dynamical properties of the solution, with direct implications in biology and electrochemistry.5−7 In terms of their effects on the structure of water, ions can be classified as kosmotropes (structure-makers) and chaotropes (structure-breakers). Kosmotropes are generally ions with high charge density which have the ability to order the surrounding water molecules, while chaotropes are ions which disrupt the hydrogen-bond network of liquid water. However, the extent to which specific ions affect the structure of water beyond the first hydration shell is a topic of much debate in current literature.1,8,9 Several experimental and theoretical studies have focused on characterizing the molecular mechanisms that drive ion hydration from the gas phase10−16 to condensed phases and interfaces.17−22 Theoretical studies are often constrained by either the accuracy of the force fields used in computer simulations or the high cost associated with correlated electronic structure calculations. On the other end, the analysis of X-ray and neutron diffraction measurements is often not © 2016 American Chemical Society
straightforward, relying on the adoption of an underlying molecular model for the postprocessing of the raw data.1,23,24 Furthermore, resolving the different features observed in linear and nonlinear vibrational spectra into specific structural and dynamical behavior of the water molecules poses substantial difficulties due to the complex nature of the underlying hydrogen-bond network.25 Despite recent progress, a unified picture of ion hydration remains elusive, requiring a quantitative assessment of the competing enthalpic and entropic effects associated with the interplay between water−water and ion−water interactions in different environments. To derive a molecular-level understanding of ion hydration, we have recently undertaken the development of full-dimensional ion−water potential energy functions (called MB-nrg for Many-Body-energy) which are derived entirely from correlated electronic structure data with the goal of accurately and systematically describing the properties of ion-containing aqueous systems from the gas phase to the condensed phase. The MB-nrg potentials are built upon the many-body expansion of the total energy (MBE)26 Received: March 24, 2016 Published: May 4, 2016 2698
DOI: 10.1021/acs.jctc.6b00302 J. Chem. Theory Comput. 2016, 12, 2698−2705
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