Theoretical study on ground and low-lying excited electronic states of BrCl: Ab initio MRCI(SD)+Q calculations

Ab initio calculations on the ground and low-lying electronic states of the BrCl molecule have been carried out within the framework of the internally contracted multireference singles and doubles configuration interaction method by using the all-electronic basis sets cc-pVTZ-DK with Davidson size-extensively correction and Douglas-Kroll-Hess scalar relativistic correction. The spin-orbit coupling interactions are further taken into account through the state interaction approach with the full Breit-Pauli operator. The 23 Ω states generated from the 12 Λ − S states are studied. The spectroscopic constants of the bound states are derived by fitting the calculated potential energy curves. The calculated potential barrier of B3Π0+ is located at the internuclear distance R of 3.261 Å and lies on 236.8 cm−1 above the first dissociation limit, consistent with experimentally the potential barrier height (275 cm−1) and the intersection location R (3.26 Å). The spin-orbit coupling between the B3Π0+ state and the essentially unbound state Y0+ is investigated. It is shown that the B3Π0+ state is significantly mixed with another 3Π0+ state as well as 3Σ0+- state, and the former is dominant. On the basis of the selection rule of the spin-orbit coupling and analysis of the Ω state components, the intersection of the repulsive C 1Π1 state with the attractive branch of the B3Π0+ potential curve near the level of the first molecular dissociation limit is confirmed to be forbidden for the spin-orbit coupling, and theoretically it is further suggested that the possible rotation-electronic coupling between B3Π0+and C 1Π1 is responsible for the spontaneous predissociation of the B3Π0+ state. The basis set superposition errors for the calculated spectroscopic constants are examined. Also the effects of the basis set and the theoretical methods on the calculated dissociation energy of the ground state are briefly discussed.