Effect of basis set superposition error on the water dimer surface calculated at Hartree-Fock, Moller-Plesset, and density functional theory levels

Counterpoise (CP) corrected optimizations of water dimer using the D95++(d,p) basis set at various levels of MP and DFT are presented. The MP2(full) calculations predict an interaction energy of -4.75 kcal/mol, close to the "MP2 limit" and to reported MP2 calculations with much large basis sets. DFT (B3PLYP) CP-corrected optimization provided the best interaction energy (-5.22 kcal/mol). The optimized O ... O separations are larger on the CP-optimized than on the uncorrected surface, as expected. These increases improve the separations predicted by DFT and worsen those predicted by the MP methods. However, the MP2 surface is so flat that the distortions of the O ... O separations needed to make the calculations agree with the experimental value require less than 0.1 kcal/mol.