Energy decomposition analysis (EDA) is a well-established
approach
to dissect the interaction energy into chemically sound components.
Despite the inherent requirement of reference states has been a long-standing
object of debate, the direct relation with the molecular orbital analysis
helps in building up predictive models. The alternative molecular
energy decomposition schemes that decompose the total energy into
atomic and diatomic contributions, such as the interacting quantum
atoms (IQA), has no external reference requirements and also the intra-
and intermolecular interactions are treated on equal footing. However,
a connection with heuristic chemical models are limited, bringing
about a somewhat narrower predictive power. While efforts to reconcile
the bonding picture obtained by both methodologies have been discussed
in the past, asynergic combination of them has not been tackled
yet. Herein, we present the use of IQA decomposition of the
individual terms arising from the EDA in the context of intermolecular
interactions, henceforth EDA–IQA. The method is applied to
a molecular set covering a wide range of interaction types, including
hydrogen bonding, charge–dipole, π–π and
halogen interactions. We find that the electrostatic energy from EDA,
entirely seen as intermolecular, leads to meaningful and non-negligible
intra-fragment contributions upon IQA decomposition, originated from
charge penetration. EDA–IQA also affords the decomposition
of the Pauli repulsion term into intra- and inter-fragment contributions.
The intra-fragment term is destabilizing, particularly for the moieties
that are net acceptors of charge, while the inter-fragment Pauli term
is actually stabilizing. In the case of the orbital interaction term,
the sign and magnitude of the intra-fragment contribution at equilibrium
geometries is largely driven by the amount of charge transfer, while
the inter-fragment contribution is clearly stabilizing. EDA–IQA
terms show a smooth behavior along the intermolecular dissociation
path of selected systems. The new EDA–IQA methodology provides
a richer energy decomposition scheme that aims at bridging the gap
between the two main distinct real-space and Hilbert-space methodologies.
Via this approach, the partitioning can be used directionally on all
the EDA terms aiding in identifying the causal effects on geometries
and/or reactivity.