MRCI study of spectroscopic and molecular properties of X2Πg, a 4Πu, A2Πu, b 4 , D2Δg and B2 electronic states of ion

Abstract

The potential energy curves (PECs) of six low-lying electronic states (X2Πg, a 4Πu, A2Πu, b 4 , D2Δg and B2 ) of ion were studied by the ab initio quantum chemical method. The calculations were carried out with the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with large correlation-consistent basis sets. Effects on the PECs of the core–valence correlation and relativistic corrections are taken into account. The way to consider the relativistic correction is to use the second-order Douglas–Kroll Hamiltonian (DKH2) approximation. The core–valence correlation correction is carried out with the cc-pCVQZ basis set, and the relativistic correction is performed at the level of cc-pVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI + Q). These PECs are extrapolated to the complete basis set (CBS) limit by the two-point total-energy extrapolation scheme. With these PECs, the spectroscopic parameters (Te , De , D 0, Re , ωe , ωexe , αe and Be ) are determined and compared with those reported in the literature. The conclusion can be reached that the effect on the spectroscopic parameters of the core–valence correlation correction is larger than that of the relativistic correction. With the PECs obtained by the MRCI + Q/CV+DK+56 calculations, the vibrational levels and inertial rotation constants of the first 26 vibrational states are determined for these electronic states of non-rotating ion. Comparison with the experimental data shows that the present spectroscopic parameters and molecular constants are accurate.