Computational studies of the hydroxyapatite nanostructures, peculiarities and properties

In this review paper the main approaches to modeling the hydroxyapatite (HAP) structures and first-principle calculations of their properties, pure and with various defects, are considered. First, the HAP nano-particles (NPs) and clusters peculiarities are described using different methods: molecular mechanical and quantum mechanical, especially semi-empirical such as PM3. Both approximations used here, namely, restricted Hartee-Fock (RHF) and unrestricted Hartee-Fock (UHF), are considered. The influence of the protons (hydrogens), contained in the surrounding medium (pH), on the formation of HAP nanoparticles of various sizes and shapes is considered and discussed. Second, the HAP crystal unit cells studies are considered on the basis of a density functional theory (DFT) modelling. The main peculiarities of both phases (hexagonal and monoclinic) are considered too, including their ordered and disordered substructures. One of the important aspects of the computer modeling of HAP is to build the models and consider various structural modifications of HAP (such as, vacancies of oxygen atoms and hydroxyl OH group, hydrogen interstitials and different substitutions of atoms in HAP unit cell), which allow explicitly creating and exploring the changes in the charges of HAP and the electrical potential on the HAP surface. HAP modifications are most close to biological HAP and therefore are necessary for implant medical applications and can create and functionalize HAP surface with most adhesive properties for living cells (osteoblasts, osteoclatst). This improves the HAP implant quality. Besides, it has recently been established that oxygen vacancy in HAP influences their photo-catalytic properties. It is important for HAP usage as in environmental remediation and for bacteria inactivation. Therefore it is very important to create and investigate the oxygen vacancy models in HAP, and others defects models. In this work we review a DFT modelling and studies of HAP, both pure perfect bulk and imperfect bulk cases. Special HAP modelling approaches are used for layered slab super-cells units, which include vacuum spaces between the layered slabs forming HAP surface. To all these computer studies the first principle calculations were applied. In this review various DFT approximations are analysed for bulk and surface modified HAP. These approximations are carried out using both the local basis (local density approximation – LDA, in AIMPRO codes) and the plane-waves (generalized gradient approximation – GGA, in VASP codes). Data of all structures and models of HAP defects investigated are widely analyzed.