Computer simulation of stoichiometric hydroxyapatite: Structure and substitutions

A force field for the modeling of hydroxyapatite; Ca10(PO4)6OH2 (HAP) is established based upon transferable potentials. Ca–O, P–O and O–O potentials were transported from those previously published for fluorapatite and based on single crystal experimental data. The interactions of hydroxyl oxygen with calcium and phosphate were re-scaled by fitting to experimental data for CaO and AlPO4, respectively, to account for the reduction in the oxygen charge from −2.0 to −1.426. Force field accuracy is tested by comparing the calculated and experimental values for the cell constant and atom positions in the unit cell. The elastic constants and bulk modulus calculated for HAP are in close agreement with the experimental results. The potentials were also used to calculate the compressibility data of HAP and fluorapatite, and these results also agree with the published experimental data. Using formal charges for metal cations allows modeling the complete solid solution of Cd–Ca hydroxyapatite with a good accuracy.