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The electron structure of the typical calcium phosphate ceramic, hydroxyapatite (OHAP), is investigated by the use of the tight-binding recursion method. The local density of states, partial density of states, structural energy and charge transfer are calculated, as well as the interatomic energy between atoms and bond order integral. Some physical and chemical properties of OHAP are discussed. The calculated results show that both the OH group and the PO4 group can be regarded as individual structure units in OHAP owing to strong interaction between atoms. Meanwhile, the atomic structure of a grain boundary is determined on the basis of the near-coincidence site lattice model with consideration of the bond length in the crystal and the lattice distortion. The electron structure and energy of a Sigma = 7 grain boundary are calculated. The results show that a grain boundary with a slight lattice misfit has a lower energy. [ABSTRACT FROM AUTHOR]