Structural and vibrational features of hexagonal hydroxyapatite HA [Ca₁₀(PO₄)₆(OH)₂, space group P6₃] 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 Γ 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 P2₁/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.

Hydroxyapatite is the mineral component of human bones and teeth enamel and is used as synthetic biomaterial. It also grows outside bioglasses as a response of their incorporation in body fluids. The focus is then on understanding the microscopic steps occurring at its surfaces as this allows researchers to understand the key features of biomolecular adhesion. This perspective article deals with in silico simulations of these processes by quantum-mechanical methods based on density functional theory using the hybrid B3LYP functional and Gaussian basis functions.