Completely Multipolar Model for Many-Body Water-Ion and Ion-Ion Interactions

TL;DR

This paper introduces the Completely Multipolar Model (CMM), an advanced force field that accurately captures many-body interactions in aqueous ion solutions, including polarization and covalency effects, improving predictions of ion behavior.

Contribution

The CMM model incorporates environment-dependent atomic polarizabilities and multipoles, providing a more accurate and comprehensive description of ion interactions in water.

Findings

CMM accurately reproduces EDA energy terms for ion interactions.

CMM captures covalency effects and polarization damping.

CMM performs well on vibrational and cluster benchmarks.

Abstract

This work constructs an advanced force field, the Completely Multipolar Model (CMM), to quantitatively reproduce each term of an energy decomposition analysis (EDA) for aqueous solvated alkali metal cations and halide anions and their ion pairings. We find that all individual EDA terms remain well-approximated in the CMM for ion-water and ion-ion interactions, except for polarization, which shows errors due to the partial covalency of ion interactions near their equilibrium. We quantify the onset of the dative bonding regime by examining the change in molecular polarizability and Mayer bond indices as a function of distance, showing that partial covalency manifests by breaking the symmetry of atomic polarizabilities while strongly damping them the at short-range. This motivates an environment-dependent atomic polarizability parameter that depends on the strength of the local electric field experienced by the ions to account for strong damping, with anisotropy introduced by atomic multipoles. The resulting CMM model for ions provides accurate dimer surfaces and three-body polarization and charge transfer compared to EDA, and shows excellent performance on various ion benchmarks including vibrational frequencies and cluster geometries.