Dispersion Interactions between Molecules in and out of Equilibrium Geometry: Visualization and Analysis

The London dispersion
interactions between systems undergoing bond
breaking, twisting, or compression are not well studied due to the
scarcity and the high computational cost of methods being able to
describe both the dynamic correlation and the multireference character
of the system. Recently developed methods based on the Generalized
Valence Bond wave function, such as EERPA-GVB and SAPT­(GVB) (SAPT
= symmetry-adapted perturbation theory), allow one to accurately compute
and analyze noncovalent interactions between multireference systems.
Here, we augment this analysis by introducing a local indicator for
dispersion interactions inspired by Mata and Wuttke’s Dispersion
Interaction Density [J.
Comput. Chem. 2017, 38, 15−23] applied on top of
an EERPA-GVB computation. Using a few model systems, we show what
insights into the nature and evolution of the dispersion interaction
during bond breaking and twisting such an approach is able to offer.
The new indicator can be used at a minimal cost additional to an EERPA-GVB
computation and can be complemented by an energy decomposition employing
the SAPT­(GVB) method. We explain the physics behind the initial increase,
followed by a decrease in the interaction of linear molecules upon
bond stretching. Namely, the elongation of covalent bonds leads to
the enhancement of attractive dispersion interactions. For even larger
bond lengths, this effect is canceled by the increase of the repulsive
exchange forces resulting in a suppression of the interaction and
finally leading to repulsion between monomers.