High level ab initio calculations of intermolecular interaction of propane dimer: Orientation dependence of interaction energy

Intermolecular interaction of the propane dimer was calculated with the MP2 level electron correlation correction using several basis sets up to the cc-pVQZ. The calculated interaction energy greatly depends on the basis set. Small basis sets underestimate the attraction considerably. The effects of electron correlation beyond MP2 are not large. Intermolecular interaction energies of 23 orientations of propane dimers were calculated at the MP2 level with a large basis set including multiple polarization functions. In all dimers, the inclusion of electron correlation considerably increases the attraction. The dispersion interaction is found to be the major source of attraction, whereas the electrostatic interaction is very small. The C-2h dimer in which the two C-2 axes of propane monomers have antiparallel orientation has the largest binding energy. The separation between the two methylene carbon atoms at the potential minimum in this dimer is the shortest among the 23 dimers. The short separation, which increases the dispersion energy, is the cause of the large binding energy of the C-2h dimer. The estimated MP2 and CCSD(T) interaction energies of the propane dimer at the basis set limit are -1.99 and -1.94 kcal/mol, respectively.