Assessing Ion–Water Interactions in the AMOEBA Force Field Using Energy Decomposition Analysis of Electronic Structure Calculations

Abstract

AMOEBA is a molecular mechanics force field that addresses some of the shortcomings of a fixed partial charge model, by including permanent atomic point multipoles through quadrupoles, as well as many-body polarization through the use of point inducible dipoles. In this work, we investigate how well AMOEBA formulates its non-bonded interactions, and how it implicitly incorporates quantum mechanical effects such as charge penetration (CP) and charge transfer (CT), for water-water and water-ion interactions. We find that AMOEBA's total interaction energies, as a function of distance and over angular scans for the water dimer and for a range of water-monovalent cations, agree well with an advanced density functional theory (DFT) model, whereas the water-halides and water-divalent cations show significant disagreement with the DFT result, especially in the compressed region when the two fragments overlap. We use a second-generation energy decomposition analysis (EDA) scheme based on absolutely localized molecular orbitals (ALMOs) to show that in the best cases AMOEBA relies on cancellation of errors by softening of the van der Waals (vdW) wall to balance permanent electrostatics that are too unfavorable, thereby compensating for the missing CP effect. CT, as another important stabilizing effect not explicitly taken into account in AMOEBA, is also found to be incorporated by the softened vdW interaction. For the water-halides and water-divalent cations, this compensatory approach is not as well executed by AMOEBA over all distances and angles, wherein permanent electrostatics remains too unfavorable and polarization is overdamped in the former while overestimated in the latter. We conclude that the DFT-based EDA approach can help refine a next-generation AMOEBA model that either realizes a better cancellation of errors for problematic cases like those illustrated here, or serves to guide the parametrization of explicit functional forms for short-range contributions from CP and/or CT.

Development of an Advanced Force Field for Water Using Variational Energy Decomposition Analysis

“…For the water dimer, the oxygen-oxygen distance is used as the scanned coordinate, where as for water trimer one of the water molecules is moved from the centroid of the triangle formed by the three oxygen atoms. 53,54 Additionally, permanent electrostatics were computed for 50 random configurations extracted from a 50 ps MD simulation at 298 K, with each configuration We have previously shown that the incorporation of atomic anisotropic polarizabilities in the MB-UCB offers significantly better agreement with the ALMO-EDA polarization for the water dimer, trimer and the MD-pentamer results than found using standard AMOEBA03. 57 Figure 1c re-examines the anisotropic polarization energy of MB-UCB for the water dimer, but this time for the two multipole truncations, in which we find that both agree very well with the ALMO-EDA polarization energy.…”

Assessing many-body contributions to intermolecular interactions of the AMOEBA force field using energy decomposition analysis of electronic structure calculations

“…In a previous article 69 , we assessed the accuracy of the non-covalent terms in AMOEBA for the water dimer and a series of water-ion dimers using a variational EDA approach based on absolutely localized molecular orbitals (ALMO) 67,70 at the highly accurate ωB97X-V/def2-QZVPPD QM level of theory 71,72 . The properties of the second generation ALMO-EDA approach that we find desirable include: (1) the energy decomposition relies on a series of intermediate wavefunctions that each obeys anti-symmetry; (2) the better handling of strongly interacting systems where the accuracy of perturbative approaches might be undermined by convergence issues; (3) the use of a high-quality density functional that adequately captures electron correlation and other effects for weakly interacting systems as benchmarked against the references obtained with high-level correlated methods (see Ref.…”

Variational Forward-Backward Charge Transfer Analysis Based on Absolutely Localized Molecular Orbitals: Energetics and Molecular Properties

“…where the frozen term can be further decomposed into contributions from permanent electrostatics (∆E ELEC ), Pauli repulsion (∆E PAULI ), and dispersion interaction (∆E DISP ). [52][53][54] Such a decomposition is usually performed at a single given geometry and thus we refer to this approach as the vertical ALMO-EDA thereafter. For details regarding the physical meaning and mathematical definition of each of these terms, we refer the reader to our previous publications.…”