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

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.