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

Abstract

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.

Molecular interactions in electron-groups embedding generalized valence bond picture

“…We have already observed a similar behavior (i.e., attraction growth accompanying a bond stretching) for other vdW complexes in Ref. [22], where we explained it by the increased polarizability of the electrons in the stretched bond region and an electron density buildup in the region between monomers. This causes a rise in both Pauli repulsion and dispersion interaction, which explains the existence of a minimum in the interaction energy along the F 2 bond stretch mode.…”

“…Already in [22], we have shown that EERPA-GVB describes accurately the dispersion-dominated systems. Here, we reaffirm this statement, adding that for this type of systems EERPA-GVB tends to be on par or even more accurate than SAPT [11,44] computations (see Table 1).…”

“…While those observations do not allow one to determine the height of the fluorination barrier energy, they do highlight the importance of accurate description of noncovalent interactions, while the molecules are attempting a reaction. Since even highly sophisticated coupled-cluster methods frequently fail at this task [22], it is unsurprising that the computed value of a reaction barrier may be inaccurate.…”

“…Adding dynamiccorrelation correction to GVB energy via perturbation theory or linearized multireference coupled-cluster theory allows one to include dispersion interaction component but the accuracy is poor (it must be admitted that the applications are scarce) [23]. EERPA, on the other hand, based on introducing extended random phase approximation correlation correction in an embedding fashion, was shown to be a tailor-made approach for describing intermolecular interactions of multireference systems [22]. It is accurate and numerically stable.…”

Recent developments in symmetry‐adapted perturbation theory

“…Interactions between genuinely multireference molecules require capturing a delicate balance between intermonomer dynamical correlation and intramonomer static correlation and are subject of intense research. For example, the stretching of a covalent bond in one of the interacting molecules has been shown to first enhance and then suppress the noncovalent binding, and single‐reference SAPT provides useful insight into this behavior up to a certain point on the stretching curve . Beyond that point, a truly multireference SAPT formalism is required, a goal that has not been fully accomplished yet.…”