  A realistic two-center shell model for fusion is proposed, which is based on
two spherical Woods-Saxon potentials and the potential separable expansion
method. This model describes the single-particle motion in a fusing system. A
technique for calculating stationary diabatic states is suggested which makes
use of the formal definition of those states, i.e., they minimize the radial
nonadiabatic coupling between the adiabatic states. As an example the system
$^{16}$O + $^{40}$Ca $\to$ $^{56}$Ni is discussed.
