Electronic Curves Crossing in Methyl Iodide by Spin--Orbit Ab Initio Calculation

An ab initio investigation of electronic curve crossing in a methyl iodide molecule is carried out using spin--orbit multiconfigurational quasidegenerate perturbation theory. The one-dimensional rigid potential curves and optimizedeffective curves of low-lying states, including spin--orbit coupling and relativistic effects, are calculated. The spin--orbit electronic curve crossing between ³Q _0⁺ and ¹Q₁, and the shadow minimum in potential energy curve of ³Q_0⁺ at large internuclear distance are found in both sets of the curves according to the present calculations. The crossing position is in the range ofR_C-I=0.2370±0.0001nm. Comparisons with other reports are presented.