Comparison of ab initioMODPOT interaction energy components against large basis set MBPT (4) calculations for nitromethane dimer

Ab-Initio MODPOT SCE calculations using our own optimized well-balanced minimal basis sets have little basis set superposition error (BSSE) and have proven over the years to give reliable intermolecular geometries and interaction energies. Another approach, using ab initio potential functions from energy partitioned ab initio calculations, has also yielded reliable interaction energies and intermolecular geometries, including crystal structures. We used both of these techniques in comparing the results of a published large all-electron basis set calculation on the nitromethane dimer with the above methods. Our SCF intermolecular minimum energy was at the same internuclear distance as the large all-electron basis set. Our SCF interaction energy, corrected for BSSE, was only 0.4 kcal above the SCF energy of the large basis set SCF calculation corrected for BSSE. We also calculated the energy partitioned components with our ab initio MODPOT basis set and with the larger all-electron basis set and showed that the small difference in interaction energy was due to a small difference in the first-order electrostatic multipolar term. (We also calculated this term from correlated wave functions [SDQ-MBPT (4)] using both ab initio MODPOT and the larger all-electron basis sets). From values of the first-order electrostatic multipolar term from SCF and correlated monomer wave functions, the contribution to intermolecular interaction energy due to the use of correlated monomer wave functions has been estimated. Our results indicate either the dominant role of electrostatic term or near cancellation of the remaining components of intermolecular interaction energy. For cyclic nitromethane dimer at equilibrium distance these effects approach 0.9 kcal/mol (nearly 1/4 of total interaction energy). In addition, we showed that the semitheoretical expression and parameters we use for estimating the dispersion energy gave results very close to the published variation perturbation results from the larger basis set calculation.