  We theoretically address grazing incidence fast atom diffraction (GIFAD) for
H atoms impinging on a LiF(001) surface. Our model combines a description of
the H-LiF(001) interaction obtained from Density Functional Theory calculations
with a semi-quantum treatment of the dynamics. We analyze simulated diffraction
patterns in terms of the incidence channel, the impact energy associated with
the motion normal to the surface, and the relevance of Van der Waals (VdW)
interactions. We then contrast our simulations with experimental patterns for
different incidence conditions. Our most important finding is that, for normal
energies lower than 0.5 eV and incidence along the <100> channel, the inclusion
of Van der Waals interactions in our potential energy surface yields a greatly
improved accord between simulations and experiments. This agreement strongly
suggests a non-negligible role of Van der Waals interactions in H/LiF(001)
GIFAD in the low-to-intermediate normal energy regime.
