Periodic ab initio study of structural and vibrational features of hexagonal hydroxyapatite Ca10(PO4)6(OH)2

Structural and vibrational features of hexagonal hydroxyapatite HA [Ca-10(PO4)(6)(OH)(2), space group P6(3)] are computed ab initio within a periodic approach using the CRYSTAL03 program and the B3LYP hybrid functional with a Gaussian-type basis set of polarized double zeta quality. Experimental lattice parameters and internal coordinates have been fully optimized and the final structure characterized by means of its band structure, density of states and Mulliken analysis. The full B3LYP harmonic vibrational spectrum of HA at G point has also been computed and compares well with the available experimental IR and Raman data. Nevertheless, the presence of one negative frequency in the computed spectrum shows that, within the hexagonal symmetry imposed by the P63 group, the structure is a saddle point. This is at variance with the monoclinic structure (under P2(1)/b space group), which has been computed, with the same approach, to be a minimum of 17 kJ mol(-1) (per unit cell) more stable than the corresponding hexagonal HA structure.