Basis set superposition error in MP2 and density-functional theory:A case of methane-nitric oxide association

A systematic study of basis set superposition error (BSSE) behavior in H 3 C - H ... [ N O ] complexes for both -H...N- and -H...O- orientations were carried out using MP2 and density-functional theory with Pople's [ 6 - 31 G ( d , p ) , 6 - 311 + + G ( n d , n d ) , where n = 1 , 2 , 3 , and 6 - 311 + + G ( 3 d f , 3 p d ) ] and Dunning's augmented correlation consistent basis sets [aug-cc- pV X Z ( X = D and T)]. Corrected and uncorrected counterpoise potential-energy surfaces (PESs) were explored and differences obtained between them indicate that reliable optimizations of these molecular interactions must be carried out in a PES free of BSSE, even in the case of large basis sets and popularly used functionals such as B3LYP. Although all basis used could be always considered within a margin of approximation for representing molecular orbitals and show important values of BSSE, 6 - 311 + + G ( 2 d , 2 p ) basis set shows the best results in uncorrected PES with respect to the corrected ones. B3LYP functional produces erratic results: complexes appear repulsive and the intermolecular distances are always large, evidencing the lack of a correct dispersive forces treatment in the original parameterization. According to the MP2 results, the -H...N- interactions appear as slightly more stable than those of the -H...O- orientation.