Ab initio study of nonadiabatic coupling matrix elements between excited 2(2)A ' and 3(2)A ' electronic states of C2H

The conical intersection (CI) between the 2(2)A' and 3(2)A' electronic stat es of C2H occurring at the C-2v-symmetric geometry has been characterized b y line integral calculations using ab initio nonadiabatic matrix coupling e lements. The calculations confirmed that when a path surrounds this CI and does not approach, too closely, CIs formed by adjacent states, the line int egral produces for the topological phase, the expected pi -value. Increasin g 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 pres ent one. (C) 2001 Elsevier Science B.V. All rights reserved.