  We discuss the feasibility of the embedded cluster approach for it ab-initio
calculations of charge exchange between ions and a LiF surface. We show that
the discrete density of valence states in embedded clusters converges towards
the continuum limit of the density of states in the valence band of an
infinitely extended LiF surface. Screening of the holes that are left in the
surface after electron transfer to the projectile plays an important role for
the correct level ordering in the calculation of potential energy surfaces. We
discuss to which extent the hole screening is taken into account by different
levels of approximations which are customarily employed in quantum chemistry.
The central result of the paper is the convergence of potential energy curves
with respect to cluster size: Out of the increasing number of potential energy
curves (converging towards a continuum for infinite cluster size), only a small
number of states effectively interacts with the capture level of the projectile
and determines the charge transfer efficiency.
