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

The geometries and harmonic frequencies of H2O and HF have been comput ed 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) calcul ations with an 'augmented' spdfg basis set are in close agreement with experiment; addition of h functions does not substantially affect res ults, indicating convergence with respect to angular momentum. Core co rrelation increases stretching frequencies by 4-7 cm-1; accounting for residual inaccuracies in the correlation treatment and contraction er ror lowers them by about 6 and 3 cm-1, respectively. The remaining sma ll errors are due to relativistic and nonadiabatic effects, in that or der of importance. Calculations on BH indicate that, as expected, the augmenting functions can safely be omitted on elements that are not hi ghly electronegative.