Pseudopotential local-spin-density studies of neutral and chargedMgn(n≤7) clusters

Electronic-structure calculations on neutral, anionic, cationic, and doubly ionized ${\mathrm{Mg}}_{\mathit{n}}$ (n≤7) clusters have been carried out to examine the effect of charging on the geometry and stability of clusters. Our studies employ Gaussian basis functions, treat exchange-correlation effects via the local-spin-density approximation, and use pseudopotentials to replace the cores. For neutral clusters we focus on the evolution of the nature of the electronic states as a function of size, and show that within an s-p–mixing criterion ${\mathrm{Mg}}_7$ is not metallic. We find that the neutral clusters have compact equilibrium geometries, and that for n>4 the ground-state geometries of the anionic and cationic clusters are obtained from the neutral clusters by small displacements of the atoms. Both cationic and anionic clusters are more stable than the corresponding neutral cluster, the mechanism by which the electron in anionic and the hole in cationic clusters enhance their stability is discussed. Our studies on doubly ionized clusters show that their geometries are dictated by the Coulomb repulsion between the holes and the electronic bonding forces. ${\mathrm{Mg}}_7^{2+}$ is the first doubly charged cluster whose equilibrium geometry is not a linear chain. All doubly charged cations are metastable, their dissociation barrier is calculated. We also present a complete study of the Born-Oppenheimer surfaces of neutral and ionized trimers, and discuss the importance of geometrical constraints on the ionization process.