Non-covalent interactions using local correlation methods: energy partitioning, geometry optimisation and harmonic frequency calculations

Abstract

The performance of density fitting, local correlation methods (DF-LMP2 and DF-LCCSD) in studies of non-covalent interactions is tested against literature data for a standard set of 22 inter-molecular complexes. Partitioning of interaction energy in the local correlation approach, based on the classes of occupied and virtual orbital involved in the interaction, clearly distinguishes the three types of interaction present in the set of complexes, in agreement with previous classifications. Geometry optimisation is found to be straightforward with DF-LMP2 without the need for counterpoise correction, resulting in geometries very close to previous, counterpoise-corrected structures. Spin-component scaling of gradients to correct for the known shortcomings of conventional MP2 has only a small effect on geometries in most cases, but significantly alters the distance between aromatic rings in stacked complexes. Harmonic frequency calculation is made possible by efficient use of parallel computing resources, and confirms all structures to be true minima, unlike previous estimates using density functional theory. Corrections for the change in zero-point vibrational energy are determined from this data, and typically constitute between 10 and 50% of the overall binding energy of the complex.