A comparison of the B3LYP and MP2 methods in the calculation of phosphate complexes

Hydrogen-bond complexes of dihydrogen phosphate and dimethyl phosphate anions with orthosilicic acid, water, and ionic complexes with sodium cations are investigated with the density functional (B3LYP/6-31G(d,p) and B3LYP/6-31+G(d,p)) theory and the second-order perturbation (MP2/6-31+G(d,p)) theory. The geometries and IR spectra of the complexes obtained with the B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) level calculations are found to be very similar. Inclusion of diffuse functions in the basis set significantly improves the reproducibility of the experimental data. The BSSE corrections significantly reduce thr differences in stabilisation energies obtained at different levels of theory, and as expected are the largest with the least flexible basis set calculations. (C) 2000 Elsevier Science B.V. All rights reserved.