Force Decomposition Analysis: A Method to Decompose Intermolecular Forces into Physically Relevant Component Contributions

Computational quantum chemistry can be more than just
numerical
experiments when methods are specifically adapted to investigate chemical
concepts. One important example is the development of energy decomposition
analysis (EDA) to reveal the physical driving forces behind intermolecular
interactions. In EDA, typically the interaction energy from a good-quality
density functional theory (DFT) calculation is decomposed into multiple
additive components that unveil permanent and induced electrostatics,
Pauli repulsion, dispersion, and charge-transfer contributions to
noncovalent interactions. Herein, we formulate, implement, and investigate
decomposing the forces associated with intermolecular interactions
into the same components. The resulting force decomposition analysis
(FDA) is potentially useful as a complement to the EDA to understand
chemistry, while also providing far more information than an EDA for
data analysis purposes such as training physics-based force fields.
We apply the FDA based on absolutely localized molecular orbitals
(ALMOs) to analyze interactions of water with sodium and chloride
ions as well as in the water dimer. We also analyze the forces responsible
for geometric changes in carbon dioxide upon adsorption onto (and
activation by) gold and silver anions. We also investigate how the
force components of an EDA-based force field for water clusters, namely
MB-UCB, compare to those from force decomposition analysis.