INTERMOLECULAR INTERACTION POTENTIAL OF THE CARBON-DIOXIDE DIMER

The energy profile of the lowest energy path connecting the slipped pa rallel (C-2h) and T-shaped (C-2 upsilon) dimers of carbon dioxide was investigated using a reasonably large 6-311+ G(2df) basis set with the MP2 level electron correlation correction and with the counterpoise c orrection, There was no inversion barrier and the T-shaped dimer was a transition state connecting the two slipped parallel dimers. The calc ulated interaction energies of the two dimers (-1.290 and -1.072 kcal/ mel) were larger than those reported previously. The calculated energ y difference of the two dimers (0.218 kcal/mol) was also larger than t he previous values (less than 0.1 kcal/mol). The effects of basis set, electron correlation, and basis set superposition error were studied in detail. The second-order Moller-Plesset (MP2) interaction energies of the two dimers were close to the coupled cluster calculations using single and double substitutions including noniteratively triple excit ations [CCSD(T)] ones. The choice of the basis set greatly affected th e calculated interaction energies. Small basis sets underestimated the attractive interaction energies. Diffuse polarization functions were essential to evaluate the attractive interaction. The calculated MP2 i nteraction energies of the two dimers with the counterpoise correction increased by the augmentation of the diffuse polarization functions t o the 6-311G basis set, These values were close to the estimated MP2 interaction energies at the complete basis set (-1.362 and -1.140 kcal /mol), The 6-311+ G(2df) basis set was reasonably large to evaluate th e attractive interactions. Although the basis set augmented with diffu se polarization functions had large basis set superposition error (BSS E), the calculated interaction energies with the counterpoise correcti on were close to those calculated with the nearly BSSE free cc-pVSZ an d aug-cc-pVQZ basis sets and to the expected values at the complete ba sis set. (C) 1998 American Institute of Physics.