Theoretical investigations of small multiply charged cations. III. NeN ²⁺

First-order configuration-interaction calculations based on 4-6 σ, 1-2 π complete active space self-consistent-field reference wave functions are reported for the potential-energy curves of the 21 lowest-lying electronic states of NeN²⁺ which dissociate into (²P)Ne⁺ + (³P)N⁺ or (²P)Ne⁺ + ( ¹D)N⁺. Using the same complete active space self-consistent-field SCF reference function, second-order configuration- interaction wave functions have been calculated for the X ²Π ground state of NeN²⁺. At this level the potential minimum lies 4.57 eV above the (³P)N⁺ + (²P)Ne⁺ dissociation limit and there is a barrier to dissociation of 0.94 eV. All other states of NeN²⁺ are either repulsive, or exhibit a flat potential curve at some closer interatomic distance. In addition, the X ³Σ^- ground-state potential-energy curve of single charged NeN⁺ was computed at the complete active space SCF + second-order configuration-interaction level. The dissociation energy is predicted as D_e = 0.47 eV at r_e = 3.30 a₀. Bonding in NeN²⁺ and NeN⁺ is discussed in terms of donor-acceptor interactions between Ne and N²⁺ or N⁺, respectively. [For paper II of this series see W. Koch and G. Frenking, J. Chem. Phys. 86, 5617 (1987).]