Total-energy calculations for crystalline approximants of quasicrystalline structures: Occupation of the centers of the icosahedral units

Motivated by recent positron-annihilation experiments on quasicrystalline materials, we have investigated whether the centers of packing units [of Mackay's icosahedron for the Al-Mn system and cuboctahedron for the Mg-Zn (Al) system] are empty or filled. Our pseudopotential-based total-energy calculations suggest that the centers are occupied, in agreement with experimental positron-annihilation results. Possible reasons for discrepancies with the diffraction results are discussed.