An ab initio model for handling the Renner-Teller effect in tetra-atomic molecules. II. Study of the crossing of potential surfaces

In the present paper, we show that the Renner-Teller effect in tetra-atomic molecules is always accompanied by (avoided) crossings of potential surfaces corresponding to the electronic states that correlate with a spatially degenerate species of linear molecule. We restrict ourselves to Π electronic states and the lowest-order (harmonic) approximation. It is demonstrated that the simple model we employ, implicitly involves a diabatic transformation of the electronic states, which makes possible to remove the difficulties caused by the breakdown of the Born-Oppenheimer approximation at nonlinear as well as linear nuclear arrangements. The reliability of the model is illustrated on example of the X2Π electronic state of the HCCS radical. Numerical computations are carried out employing the data obtained in extensive ab initio calculations.