Exploring the influence of (n - 1)d subvalence correlation and of spin-orbit coupling on chalcogen bonding

This article presents a comprehensive computational investigation into chalcogen bonding interactions, focusing specifically on elucidating the role of subvalence (n - 1)d and (n - 1)sp correlation. The incorporation of inner-shell (n - 1)d correlation leads to a decrease in interaction energies for chalcogen-bonded systems (at least those studied herein), contradicting the observations regarding halogen bonding documented by Kesharwani et al. in J. Phys. Chem. A, 2018, 122 (8), 2184-2197. The significance of (n - 1)sp subvalence correlation appears to be lower by an order of magnitude. Notably, among the various components of interaction energies computed at the PNO-LCCSD(T) or DF-CCSD levels, we identify the PNO-LMP2 or DF-MP2 component of the (n - 1)d correlation as predominant. Furthermore, we delve into the impact of second-order spin-orbit coupling (SOC2) on these interactions. The SOC2 effects appear to be less significant than the (n - 1)d correlation; however, they remain non-trivial, particularly for Te complexes. For the Se complexes, SOC2 is much less important. Generally, SOC2 stabilizes monomers more than dimers, resulting in reduced binding of the latter. Notably, at equilibrium and stretched geometries, SOC2 and (n - 1)d destabilize the complex; however, at compressed geometries, they exhibit opposing effects, with (n - 1)d becoming stabilizing.