[en] We have studied the He-Si(100) scattering potential by considering the charge densities associated with different tilted dimer model structures. We demonstrate the inadequacy of spherical charge superposition schemes and develop an alternative procedure [modified atomic charge superposition (MACS)] which is calibrated against self-consistent linear-augmented-plane-wave charge densities. We demonstrate the transferability of the underlying MACS parameters for different periodicities of tilted dimer models of the Si(100) surface. We show that tilted dimer models of the Si(100) surface are consistent with experimental He diffraction data only if they are in an alternating c(4 x 2) array