Repulsive interatomic potentials for noble gas bombardment of Cu and Ni targets

Interatomic potentials that are relevant for noble gas bombardment of Cu and Ni targets have been calculated in the energy region below 10 keV. Potentials are calculated for the diatomic species: NeCu, ArCu, KrCu, Cu2, ArNi, Ni2 and NiCu. The calculations primarily employ density functional theory (with the B3LYP exchange–correlation functional). Potential curves derived from Hartree–Fock theory calculations are also discussed. Scalar relativistic effects have been included via the second-order Douglas–Kroll–Hess (DKH2) method. On the basis of a variational argument, it can be shown that the predicted potential curves represent an upper limit to the true potential curves. The potentials provide a basis for assessing corrections required to the ZBL and Molière screened Coulombic potentials, which are typically found to be too repulsive below 1–2 keV. These corrections significantly improve the accuracy of the sputter yield predicted by molecular dynamics for Ni(1 0 0), whereas the sputter yield predicted for Cu(1 0 0) is negligibly affected. The validity of the pair potential approximation in the repulsive region of the potential is tested by direct calculation of the potentials arising from the interaction of either an Ar or Cu atom with a Cu3 cluster. The pairwise approximation represents the Ar–Cu3 potential energy function with an error <3 eV at all Ar–Cu3 separations. For Cu–Cu3, the pairwise approximation underestimates the potential by ca. 10 eV when the interstitial atom is located near the centre of the cluster.