An ab initio calculation of the vibronic energies of the CH2+ molecule

In this paper we report the results of an ab initio calculation of vibronic (i.e., N = 0) energy levels of the CH2+ molecular ion giving the lowest 19 levels of the ground [Formula: see text] electronic state and the lowest 15 of the first excited Ã2B1 state. The two electronic states become degenerate (2Π) at the linear configuration and their rovibronic levels interact via the Renner effect. The ab initio calculation of the two potential surfaces was performed at 96 nuclear geometries for the ground electronic state, covering energies up to 11 000 cm−1 above equilibrium, and at 86 geometries for the excited state, covering energies up to 30 000 cm−1 above equilibrium. The multireference configuration interaction (MRCI) level of theory was used with molecular orbital bases that were optimized separately for each state by complete-active-space-self-consistent-field (CASSCF) calculations. The vibronic energy levels were calculated variationally from the potential surfaces using the Morse oscillator rigid bender internal dynamics Hamiltonian modified to include the effects of the Renner interaction. We also present calculated vibronic energies for the CD2+ and CHD+ isotopically substituted molecules.