BASIS-SET CONVERGENCE FOR GEOMETRY AND HARMONIC FREQUENCIES - ARE H-FUNCTIONS ENOUGH

The geometries and harmonic frequencies of H2O and HF have been computed using a systematic sequence of 'correlation consistent' basis sets. Even basis sets of spdfgh quality fail to reproduce the omega, of HF unless they are augmented with special anion functions. CCSD(T) calculations with an 'augmented' spdfg basis set are in close agreement with experiment; addition of h functions does not substantially affect results, indicating convergence with respect to angular momentum. Core correlation increases stretching frequencies by 4-7 cm-1; accounting for residual inaccuracies in the correlation treatment and contraction error lowers them by about 6 and 3 cm-1, respectively. The remaining small errors are due to relativistic and nonadiabatic effects, in that order of importance. Calculations on BH indicate that, as expected, the augmenting functions can safely be omitted on elements that are not highly electronegative.