Toward chemical accuracy in the description of ion–water interactions through many-body representations. Alkali-water dimer potential energy surfaces

Abstract

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.

Chemical accuracy in modeling halide ion hydration from many-body representations

“…short 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 MB-pol 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.…”

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

“…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.…”

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.…”

“…73,74 Continuing our systematic efforts aimed at developing an accurate, molecular-level description of hydration phenomena in different environments and under different thermodynamic conditions, in this study, we investigate the structure, and the temperature-dependent isomeric equilibria and vibrational spectra of M + (H 2 O) n clusters with M = Li, Na, K, Rb, and Cs, and n = 1-3, at both classical and quantum levels using the MB-nrg PEFs of Ref. 71. The hydration of alkali metal ions has been extensively studied both experimentally 75 and theoretically.…”