Further analysis and comparative study of intermolecular interactions using dimers from the S22 database.

Accurate MP2 and CCSD(T) complete basis set (CBS) interaction energy curves (14 points for each curve) have been obtained for 20 of the dimers reported in the S22 set and analytical Morse curves have been fitted that can be used in developing updated density functional theory (DFT) and force field models. The magnitude and the effect of the basis set superposition error (BSSE) were carefully investigated. We found that going up to aug-cc-pVDZ and aug-cc-pVTZ basis sets is enough to obtain accurate CBS MP2 energies when BSSE corrected values are used but aug-cc-pVTZ and aug-cc-pVQZ basis sets are needed when the BSSE uncorrected total energies are used in CBS extrapolations. MP2 interaction energies with smaller basis sets such as 6-31G* are found to have very little dispersion energy and that the true source of dispersion attributed attractive interactions is almost entirely due to BSSE. MP2 and CCSD(T) CBS interaction energies are found to be very close to one another if aromatic systems are not involved. Comparative analyses have been performed with semiempirical and ab initio methods utilizing the moderate in size but affordable 6-31G* basis set both of which can be readily applied to macromolecular systems. The new M06-2X and M06-L DFT functionals were found to be more accurate than all methods tested herein. Interaction energy curves using the SG1 grid showed discontinuities for several of the dimer systems but this problem disappeared when finer DFT numerical grids were used.