Toward Chemical Accuracy in the Description of Ion–Water Interactions through Many-Body Representations. I. Halide–Water Dimer Potential Energy Surfaces

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-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.

Chemical Accuracy in Modeling Halide Ion Hydration from Many-Body Representations

“…Building upon the accuracy and computational efficiency demonstrated by MB-pol in predicting the properties of water across different phases [118], the MBE in Eq. 1 has recently been applied to the development of two families of MB PEFs (TTM-nrg for "Thole-type model energy" [119] and MB-nrg for "many-body energy" [120])…”

“…is expressed by a 4 th -degree permutationally invariant polynomial [97] in variables that are functions of the distances between the ion and the six sites of an MBpol water molecule. The coefficients of both 2B and 3B permutationally invariant polynomials are optimized using Tikhonov regression [124] (also known as ridge regression) and supervised learning [125] to reproduce the interaction energies calculated at the CCSD(T)-F12b level of theory in the CBS limit [120,126]. It can be demonstrated that 2B and 3B MB-nrg permutationally invariant polynomials effectively recovers quantum-mechanical effects that cannot be described by purely classical expressions (e.g., charge transfer and penetration, and Pauli repulsion) employed in classical polarizable FFs.…”

“…The CBS limit for the two-body term was achieved by extrapolating the values obtained with the aug-cc-pVTZ(-PP) and aug-cc-pVQZ(-PP) basis sets using a two-point extrapolation [71,137] as in Ref. [120]. The three-body interaction energies were calculated using the aug-cc-pVTZ(-PP) basis sets and corrected for basis set superposition error (BSSE) using the counterpoise method [138].…”

Isomeric Equilibria, Nuclear Quantum Effects, and Vibrational Spectra of M+(H2O)n=1−3 Clusters, with M = Li, Na, K, Rb, and Cs, Through Many-Body Representations

“…69 Building upon the accuracy of MB-pol for water, many-body PEFs (called MB-nrg for "many-body energy") have recently been introduced to describe halide-water and alkali metal ion-water interactions. 70,71 Derived entirely from electronic structure data obtained at the coupled cluster level with single, double, and perturbative triple excitations, i.e., CCSD(T), the current gold standard for chemical accuracy, these MB-nrg PEFs have been shown to outperform both more advanced, polarizable FFs and existing DFT models in the description of the lower-order, two-body (2B) contributions to the corresponding interaction energies. 70,71 When employed in full-dimensional quantum calculations for X − (H 2 O) and X − (D 2 O) dimers, with X = F, Cl, Br, and I, the MB-nrg PEFs predict vibrational spectra in close agreement with the available experimental data, correctly reproducing anharmonic, nuclear quantum effects, and tunneling splittings.…”

“…70,71 Derived entirely from electronic structure data obtained at the coupled cluster level with single, double, and perturbative triple excitations, i.e., CCSD(T), the current gold standard for chemical accuracy, these MB-nrg PEFs have been shown to outperform both more advanced, polarizable FFs and existing DFT models in the description of the lower-order, two-body (2B) contributions to the corresponding interaction energies. 70,71 When employed in full-dimensional quantum calculations for X − (H 2 O) and X − (D 2 O) dimers, with X = F, Cl, Br, and I, the MB-nrg PEFs predict vibrational spectra in close agreement with the available experimental data, correctly reproducing anharmonic, nuclear quantum effects, and tunneling splittings. 72 Along the path connecting ion-water dimers to electrolyte solutions, ion-water clusters in the gas phase play an important role for understanding ion hydration since, due to their relatively small sizes, they are still amenable to high-level electronic structure calculations while, at the same time, they can be studied experimentally using high-resolution vibrational spectroscopy.…”