Ab initio study of nonadiabatic coupling matrix elements between excited 22A′ and 32A′ electronic states of C2H

The conical intersection (CI) between the 22A′ and 32A′ electronic states of C2H occurring at the C2v-symmetric geometry has been characterized by line integral calculations using ab initio nonadiabatic matrix coupling elements. The calculations confirmed that when a path surrounds this CI and does not approach, too closely, CIs formed by adjacent states, the line integral produces for the topological phase, the expected π-value. Increasing the radius of the circle was accompanied by a decrease in the topological phase indicating on the existence of other CIs in the vicinity of the present one.