Stabilization and Structure Calculations for Noncovalent Interactions in Extended Molecular Systems Based on Wave Function and Density Functional Theories

More than 20 years ago, we published in Chemical Reviews a paper entitled Intermolecular Interactions between Medium-Sized Systems. Nonempirical and Empirical Calculations of Interaction Energy: Successes and Failures. The situation in calculations of noncovalent interactions at that time can be best characterized by the question we posed at the very beginning of the review: Can quantum chemistry describe vdW (van der Waals; today we call it noncovalent) interactions as successfully as covalent interactions? Our answer then was unambiguously yes. We had good reason for an optimistic yes since we presented the first coupledcluster calculations including triple excitations for a (at that time) large complex-the water dimer. We stressed the importance of the triple excitations for noncovalent interactions, and in the section called Prospects, we wrote that significant progress is highly desirable with beyond-SCF methods, where new, more accurate and efficient procedures are developed.