Ab initio MRD-CI study of neutral and charged Ga2, Ga3 and Ga4 clusters and comparison with corresponding boron and aluminum clusters

MRD-CI calculations were performed on Ga, Ga₂, Ga₃, Ga₄ and on the corresponding positive and negative ions. In general, pseudopotentials were used, and 4s4p/2s2p basis sets withs andp diffuse functions and one or twod functions. For Ga₂, all-electron calculations were also performed. For Ga ₂ ^(±) , potential functions for ground and low-lying excited states are given. For Ga ₃ ^(±) , geometries were optimized both inC _2v andD _∞h symmetry. The lowest state of Ga ₃ ⁺ is found to be¹Σ _g ⁺ , of Ga₃ ⁴ A ₂, and of Ga ₃ ^{? 1} A ₁ (D _3h ). Ionization potentials and electron affinities of Ga₃ were evaluated. Many low-lying excited states of Ga ₃ ^(±) were found. Rhombic (D _2h , including squareD _4h ), tetrahedral (T _d ), T-shaped (C _2v ) and linear structures (D _∞h) were investigated in the search for the lowest state of Ga₄. A square-planar arrangement of the nuclei, withR _e = 5.30 a₀, was found to have the lowest energy. The other geometries lie about 0.5 eV higher. InD _2h symmetry, low-lying excited states of Ga₄, as well as ground and excited states of Ga ₄ ⁺ and Ga ₄ ^? were studied. Geometries, ionization potentials, electron affinities, atomization and fragmentation energies of Ga _n are compared with corresponding data for B _n and Al _n . Typical changes in going from first-row to third-row atoms are observed.