ENERGIES AND DERIVATIVE COUPLINGS IN THE VICINITY OF A CONICAL INTERSECTION - 3 - THE MOST DIABATIC BASIS

It is shown that in the immediate vicinity of an arbitrary conical int ersection at R-x, all the derivative coupling, except for the small pa rt due to the finiteness of the basis sets, is removable by the orthog onal transformation generated by the angle alpha(rho,theta,z) = lambda (theta)/2 + rho m(rho),(theta)/q(theta) + zm(z),(theta)/q(theta), wher e rho, theta,z are cylindrical polar coordinates centred at R-x,. Expr essions for lambda(theta), q(theta),m(rho),(theta) and m(z)(theta) are given. The implications of this result for numerical studies that (i) determine the 'most' diabatic basis using Poisson's equation and (ii) assess approximate diabatization schemes are discussed.