The repulsive interaction between two atoms at short distances is studied inorder to explore the range of validity of standard first-principles simulationtechniques and improve the available short-range potentials for the descriptionof energetic collision cascades in solids. Pseudopotentials represent theweakest approximation, given their lack of explicit Pauli repulsion in thecore-core interactions. The energy (distance) scale realistically accessible isstudied by comparison with all-electron reference calculations in some binarysystems. Reference calculations are performed with no approximations related toeither core (frozen core, augmentation spheres) or basis set. This is importantsince the validity of such approximations, even in all-electron calculations,rely on the small core perturbation usual in low-energy studies. The expectedimportance of semicore states is quantified. We propose a scheme for improvingthe electronic screening given by pseudopotentials for very short distances.The results of this study are applied to the assessment and improvement ofexisting repulsive empirical potentials.
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