Core-valence correlation effects for molecules containing first-row atoms.Accurate results using effective core polarization potentials

The accuracy of employing effective core polarization potentials (CPPs) to account for the effects of core-valence correlation on the spectroscopic co nstants and dissociation energies of the molecules B-2, C-2, N-2, O-2, F-2, CO, CN, CH, HF, and C2H2 has been investigated by comparison to accurate a ll-electron benchmark calculations. The results obtained fi-om the calculat ions employing CPPs were surprisingly accurate in every case studied, reduc ing the errors in the calculated valence D-e values from a maximum of nearl y 2.5 kcal/mol to just 0.3 kcal/mol. The effects of enlarging the basis set and using higher-order valence electron correlation treatments were found to have only a small influence on the core-valence correlation effect predi cted by the CPPs. Thus, to accurately recover the effects of intershell cor relation, effective core polarization potentials such as the ones used in t he present work provide an attractive alternative to carrying out computati onally demanding calculations where the core electrons are explicitly inclu ded in the correlation treatment.