Calculation of adiabatic and diabatic³Σ^-states of OH⁺

Abstract

Electronic structure calculations for several 3Σ- 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 Nα, the number of interacting states included in the block diagonalization. Suitable values for Nα depend on the internuclear separation, and several alternatives are compared. Further analysis of the calculations indicates that the excited oxygen (2p33s 3S) 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 2p4 and 2p33s configurations, and the diffuse 3s orbital then provides the dominant coupling with the O+ + H state.