ON THE DEFINITION OF NONADIABATIC EFFECTS IN CURVE-CROSSING PROBLEMS

When two electronic potentials present an avoided crossing, the adiaba tic approximation breaks down in the energy region near to the crossin g. In particular, the correspondence between exact energy levels of th e two-state system and the adiabatic levels of the lower and upper adi abatic potentials becomes ambiguous. This implies that the term ''nona diabatic effect,'' used for the difference between exact and adiabatic energy eigenvalues, loses its meaning in the crossing regime unless a n unambiguous way of assigning an adiabatic to an exact level is defin ed. This is important in order to investigate where nonadiabatic schem es, such as the generator coordinate approximation, fit in between the adiabatic approximation and quasi-exact approaches. (C) 1995 John Wil ey and Sons.