Structural Distortions Accompanying Noncovalent Interactions: Methane-Water, the Simplest C-H Hydrogen Bond.

Neglect of fragment structural distortions resulting from noncovalent interactions is a common practice when examining a potential energy surface (PES). Herein, we make quantitative predictions concerning the magnitude of such distortions in the methane-water system. Coupled cluster methods up to perturbative quadruples [CCSDT(Q)] were used in the structural optimizations to the complete basis set limit (using up to cc-pV6Z basis sets). Our results show that the interaction energy differences between the fully optimized and nonoptimized structures are on the order of 0.02 kcal mol-1. These findings imply that scanning the PES of a very weakly bound noncovalent system, while neglecting intramolecular distortions, is a reasonable approximation for points other than the minima.