  Energy decomposition analysis (EDA) based on absolutely localized molecular
orbitals (ALMOs) decomposes the interaction energy between molecules into
physically interpretable components like geometry distortion, frozen
interactions, polarization, and charge transfer (CT, also sometimes called
charge delocalization) interactions. In this work, a numerically exact scheme
to decompose the CT interaction energy into pairwise additive terms is
introduced for the ALMO-EDA using density functional theory. Unlike
perturbative pairwise charge-decomposition analysis, the new approach does not
break down for strongly interacting systems, or show significant
exchange-correlation functional dependence in the decomposed energy components.
Both the energy lowering and the charge flow associated with CT can be
decomposed. Complementary occupied-virtual orbital pairs (COVPs) that capture
the dominant donor and acceptor CT orbitals are obtained for the new
decomposition. It is applied to systems with different types of interactions
including DNA base-pairs, borane-ammonia adducts, and transition metal
hexacarbonyls. While consistent with most existing understanding of the nature
of CT in these systems, the results also reveal some new insights into the
origin of trends in donor-acceptor interactions.
