Calculation of adiabatic and diabatic (3)Sigma(-) states of OH+

Electronic structure calculations for several (3)Sigma(-) 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 u sing 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 deter mined by N-alpha, the number of interacting states included in the block di agonalization; Suitable values for N-alpha depend on the internuclear separ ation, and several alternatives are compared. Further analysis of the calcu lations indicates that the excited oxygen (2p(3)3s S-3) 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.