Structure and stability of the AlX and AlX- species

The electronic and geometrical structures of the ground and low-lying excit ed states of the diatomic AlX and AlX- series (X = H, Li, Be, B, C, N, O, a nd F) are calculated by the coupled-cluster method with all singles and dou bles and noniterative inclusion of triples using a large atomic natural orb ital basis. All the ground-state AlX molecules except for AlF can attach an additional electron and form ground-state AlX- anions. The ground-state Al Be-, AlB-, AlC-, AlN-, and AlO- anions possess excited states that are stab le toward autodetachment of an extra electron; AlBe- also has a second exci ted state. Low-lying excited states of all AlX but AlN can attach an extra electron and form anionic states that are stable with respect to their neut ral (excited) parent states. The ground-state AlLi-, AlBe-, AlB-, AlN-, and AlO- anions are found to be thermodynamically more stable than their neutr al parents. The most stable is AlO-, whose dissociation energy to Al+O- is 6.4 eV. Correspondingly, AlO possesses the largest electron affinity (2.65 eV) in the series. (C) 1999 American Institute of Physics. [S0021-9606(99)3 0606-1].