Intricacies of van der Waals Interactions in Systems with Elongated Bonds Revealed by Electron-Groups Embedding and High-Level Coupled-Cluster Approaches.

Noncovalent interactions between molecules with stretched intramonomer covalent bonds are a fascinating, yet little studied area. This shortage of information stems largely from the inability of most of the commonly used computational quantum chemistry methods to accurately describe weak long-range and strong nondynamic correlations at the same time. In this work, we propose a geminal-based approach, abbreviated as EERPA-GVB, capable of describing such systems in a robust manner using relatively inexpensive computational steps. By examining a few van der Waals complexes, we demonstrate that the elongation of one or more intramolecular covalent bonds leads to an enhanced attraction between the monomers. We show that this increase in attraction occurs as the electron density characterizing intramolecular covalent bonds depletes and migrates toward the region between the monomers. As the covalent intramonomer bonds continue to stretch, the intermolecular interaction potential passes through a minimum and eventually goes up. The findings resulting from our EERPA-GVB calculations are supported and further elucidated by the symmetry-adapted perturbation theory and coupled-cluster (CC) computations using methods that are as sophisticated as the CC approach with a full treatment of singly, doubly, and triply excited clusters.