Performance of the AMOEBA Water Model in the Vicinity of QM Solutes: A Diagnosis Using Energy Decomposition Analysis

The
importance of incorporating solvent polarization effects into
the modeling of solvation processes has been well-recognized, and
therefore a new generation of hybrid quantum mechanics/molecular mechanics
(QM/MM) approaches that accounts for this effect is desirable. We
present a fully self-consistent, mutually polarizable QM/MM scheme
using the AMOEBA force field, in which the total energy of the system
is variationally minimized with respect to both the QM electronic
density and the MM induced dipoles. This QM/AMOEBA model is implemented
through the Q-Chem/LibEFP code interface and then applied to the evaluation
of solute–solvent interaction energies for various systems
ranging from the water dimer to neutral and ionic solutes (NH3, NH4+, CN–) surrounded by increasing numbers of water
molecules (up to 100). In order to analyze the resulting interaction
energies, we also utilize an energy decomposition analysis (EDA) scheme
which identifies contributions from permanent electrostatics, polarization,
and van der Waals (vdW) interaction for the interaction between the
QM solute and the solvent molecules described by AMOEBA. This facilitates
a component-wise comparison against full QM calculations where the
corresponding energy components are obtained via a modified version
of the absolutely localized molecular orbitals (ALMO)-EDA. The results
show that the present QM/AMOEBA model can yield reasonable solute–solvent
interaction energies for neutral and cationic species, while further
scrutiny reveals that this accuracy highly relies on the delicate
balance between insufficiently favorable permanent electrostatics
and softened vdW interaction. For anionic solutes where the charge
penetration effect becomes more pronounced, the QM/MM interface turns
out to be unbalanced. These results are consistent with and further
elucidate our findings in a previous study using a slightly different
QM/AMOEBA model (Dziedzic et al. J. Chem. Phys. 2016, 145, 124106). The implications of these results for
further refinement of this model are also discussed.