Calculation of adiabatic and diabatic³Σ⁻states of OH⁺

Electronic structure calculations for several 3 Σ - states of OH + are reported. These states are important for the charge-exchange process O + H + ↔O + + H. Diabatic potential curves and their couplings were determined using the methodology of Pacher et al and of Domcke and co-workers, which is based on CAS-MCSCF-CI calculations followed by block diagonalization. The dimension of the matrix representation of the diabatic Hamiltonian is determined by N α , the number of interacting states included in the block diagonalization. Suitable values for N α depend on the internuclear separation, and several alternatives are compared. Further analysis of the calculations indicates that the excited oxygen (2p 3 3s 3 S) level participates in charge exchange through a two-step mechanism not previously recognized: Stark mixing of oxygen levels by a distant proton leads to an admixture of 2p 4 and 2p 3 3s configurations, and the diffuse 3s orbital then provides the dominant coupling with the O + + H state.