How good is the adiabatic basis in heavy ion collisions

The optimal choice of the basis for calculations of inner-shell excitations in the collisions of heavy ions is discussed. Starting from a variational principle which minimises the time-integrated coupling strength, a relation between a model two-centre distance xi(t) and the actual internuclear separation R(t) is derived. For 1sσ excitations in Pb-Pb collisions it is found that the optimal basis is almost identical to the adiabatic one. For higher quantum states the adiabatic treatment becomes less adequate. (author)