Potential energy function for apatites

An empirical potential energy function for fluor- and for hydroxyapatite is formulated and parametrised. The parameter optimisation involves a hierarchy of reference data and techniques comprising of quantum-chemical calculations for Coulomb interactions and intramolecular contributions, as well as experimental data and molecular dynamics simulations for the remaining nonbonded parameters. For fluorapatite both a flexible and a rigid phosphate model are derived, while for hydroxyapatite only the rigid variant is determined. Simulations with the final models reproduce the experimental crystal parameters within less than 1% deviation for a wide range of temperatures between 73 and 1273 K. In the case of flexible fluorapatite the computed and the experimental infrared spectra at 300 K agree excellently.