A COMPARATIVE-STUDY OF METHODS FOR DESCRIBING NONADIABATIC COUPLING -DIABATIC REPRESENTATION OF THE (1)SIGMA(+) (1)PI HOH AND HHO CONICAL INTERSECTIONS/

A number of established techniques for obtaining diabatic electronic s tates in small molecules are critically compared for the example of th e (X) over tilde and (B) over tilde states in the water molecule, whic h contribute to the two lowest-energy conical intersections. Integrati on of the coupling matrix elements and analysis of configuration mixin g coefficients both produce reliable diabatic states globally. Methods relying on diagonalization of dipole moment and angular momentum oper ators are shown to fail in large regions of coordinate space. However, the use of transition angular momentum matrix elements involving the (A) over tilde state, which is degenerate with (B) over tilde at the c onical intersections, is successful globally, provided that an appropr iate choice of coordinates is made. Long range damping of non-adiabati c coupling to give correct asymptotic mixing angles also is investigat ed.