  Experimental diffraction patterns produced by grazing scattering of fast
helium atoms from a Ag(110) surface are used as a sensitive tool to test both
the scattering and the potential models. To describe the elastic collision
process we employ the surface eikonal (SE) approximation, which is a
semi-classical method that includes the quantum interference between
contributions coming from different projectile paths. The projectile-surface
potential is derived from an accurate density-functional theory (DFT)
calculation that takes into account the three degrees of freedom of the
incident projectile. A fairly good agreement between theoretical and
experimental momentum distributions is found for incidence along different
low-indexed crystallographic directions.
