Optimal pi-stacking interaction energies in parallel-displaced aryl/aryl dimers are predicted by the dimer heavy atom count.

It is generally accepted that large basis sets are required for the accurate calculation of interaction energies for weakly bound aryl dimers, but it has recently been reported that MP2(full)/6-31G* energies, although inaccurate in absolute terms, are well-correlated with estimated CCSD(T)/CBS results. It is now shown that this correlation holds for MP2/aug-cc-pVDZ and SCS-MP2/aug-cc-pVDZ values. Linear regression of published CCSD(T)/CBS results with MP2 or SCS-MP2 results has been used to correct systematic errors observed with both MP2 theories, and these corrections are applied to 27 parallel-displaced aromatic dimers of interest in medicinal chemistry. The optimal computed interaction energies are found to be strongly correlated with the heavy atom counts of the aryl/aryl dimers. This relationship between heavy atom count and interaction energy also applies to a series of 12 aryl/non-aryl dimers such that a single linear regression equation accounts for all of the dimers studied.