Abstract
The potential energy surfaces of the ethylene radical cation near the 90°‐twisted form were clarified by an appropriate configuration interaction calculation. A characteristic conical intersection between the ground doublet and the first excited doublet states at a D 2d 90°‐twisted geometry was revealed. It was found that spin‐charge separation occurs suddenly near the twist angle of 90°. The effect of substitution on the shape of the potential energy surfaces is also discussed.