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

Abstract 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 r e , dissociation energies D e , and vibrational frequencies ω e 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 r e , O.1 eV for D e and 5 cm −1 for ω e , in coupled cluster calculations using extended basis sets.

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