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

The performance of large-core pseudopotentials (ECP), simulating X+ cores (X = K, Rb, Cs), and corresponding core-polarization potentials (CPP) is critically analyzed in calculations for bond lengths re, dissociation energies De, and vibrational frequencies ωe of alkali fluorides XF. It is shown that significant errors arises 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 re, O.1 eV for De and 5 cm−1 for ωe, in coupled cluster calculations using extended basis sets.