Ab initio calculations using the 6-31G**, 6-31 + G** and 6-31 ++ G** basis sets have been carried out for the hydrogen fluoride molecule and the hydrogen fluoride anion. To discuss the features of the electronic states obtained in each case an analysis of the topology of the total electronic charge density has been performed. Then, a truncated CI expansion has been used to build up the diabatic potential energy curves of hydrogen fluoride and its anion that lead to their dissociation. We conclude that special care has to be taken when basis sets including diffuse functions are used for calculating vertical electron affinities and describing dissociative electron transfer reactions. © 1992.