  The repulsive interaction between two atoms at short distances is studied in
order to explore the range of validity of standard first-principles simulation
techniques and improve the available short-range potentials for the description
of energetic collision cascades in solids. Pseudopotentials represent the
weakest approximation, given their lack of explicit Pauli repulsion in the
core-core interactions. The energy (distance) scale realistically accessible is
studied by comparison with all-electron reference calculations in some binary
systems. Reference calculations are performed with no approximations related to
either core (frozen core, augmentation spheres) or basis set. This is important
since the validity of such approximations, even in all-electron calculations,
rely on the small core perturbation usual in low-energy studies. The expected
importance of semicore states is quantified. We propose a scheme for improving
the electronic screening given by pseudopotentials for very short distances.
The results of this study are applied to the assessment and improvement of
existing repulsive empirical potentials.
