Ab-initio calculations of charge exchange in ion-surface collisions: an embedded-cluster approach

We discuss the feasibility of the embedded cluster approach for 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.