The accuracy of the pseudopotential approximation: Non-frozen-core effects for spectroscopic constants of alkali fluorides XF (X=K, Rb, Ca)

The performance of large-core pseudopotentials (ECP), simulating X(+) cotes (X=K, Rb, Cs), and corresponding core-polarization potentials (CPP) is critically anstlyzed in calculations for bond lengths r(e), dissociation energies D-e, and vibrational frequencies omega(e) of alkali fluorides XF. It is shown that significant errors arise due to both the ECP modelling of the frozen-core-valence interaction and the CPP treatment of non-frozen-core effects. Correcting for these errors, by introducing small-core ECP and l-dependent CPP, we obtain accuracies of 1 pm for r(e), 0.1 eV for D-e and 5 cm(-1) for omega(e), in coupled cluster calculations using extended basis sets.