Ab initioMg-(He0,He+,He2+) potentials and ion scattering by atomic strings and planes

Results of ab initio calculations for the interaction energy of the Mg-${\mathrm{He}}^0$, Mg-${\mathrm{He}}^{+}$, and Mg-${\mathrm{He}}^{2+}$ systems are presented, for a wide range of internuclear distances. The importance of using ab initio methods for the evaluation of accurate pair-wise potentials is discussed. A string dependence of the interaction energy on the charge state of the projectile is demonstrated. This effect, which exhibits marked deviations from the predictions of any parametric theory, is shown to be also crucial in the behavior of corresponding string and planar potentials commonly used in both experimental channeling and surface scattering studies. The role of long-range binary terms on the construction of string and planar potentials is also discussed.