Comparative ab initio studies of small tin and lead clusters

We have performed ab initio total-energy pseudopotential calculations on neutral and negatively charged Snn and Pb_n (n = 3 ? 10) clusters. The lowest energy structures have been determined for all clusters, and the stabilities of neutral clusters were investigated by comparing their evaporation energies and stability functions. Clusters with n = 7,10 were found to be most stable while the clusters with n = 8 and Pb_s were much less stable, in agreement with features of the observed mass spectra. Calculations on Sn^?_n and Pb^?_n show that both atomic and electronic structures of a neutral cluster change substantially upon charging. The densities of states of Sn^?_n clusters reproduce the main features of the experimental photoelectron spectra. The agreement is poorer for Pb^?_n clusters where the calculations underestimate the separation between energy levels which we think is due to the larger spin-orbit splitting in Pb, which was neglected in the calculations. We found that the differences between Sn and Pb clusters cannot be completely addressed without a more complete accounting of relativistic effects. The electron affinities of Sn_n and Pb_n clusters have also been calculated and the results agree fairly well with experimental values. Finally we considered Sn^2?₄ and Pb^2?₄ clusters and related the results to the formation of Zintl anions in liquid alkali-Sn and alkali-Pb alloys.