¹Institute of Mathematical Problems of Biology RAS, Keldysh Institute of Applied Mathematics of the Russian Academy of Sciences, Pushchino, Russia
²Southern Federal University, Rostov-on-Don, Russia
³Tomsk Polytechnic University, Tomsk, Russia
⁴Siberian State Medical University, Tomsk, Russia
⁵Institute of Strength Physics and Materials Science, Siberian Branch of the Russian Academy of Sciences, Tomsk, Russia

Abstract. The article presents the results of a study of the electrostatic potential and the work function of the hydroxyapatite (HAP) surface. To study the electrostatic potential and the electron work function of the HAP surface, modeling of the HAP structures and calculation of their properties using density functional theory (DFT) methods were used. To perform these simulations and calculations, a HAP supercell model was used, extended to a structure that includes a region of vacuum space near the HAP surface. The electron work function was calculated using the Quantum ESPRESSO program. To construct the model, a surface with the (001) orientation was taken, the thickness of the entire model is L ~ 26.5 Å, the thickness of the HAP substrate is ~ 12.5 Å, and the vacuum layer is ~ 14 Å. The equilibrium state of the entire model is achieved by relaxation of its surface HAP layers, leaving the internal layers at  5.2 Å with fixed atoms positions. After optimizing the initial HAP structure, surface layer modifications (substitution of Ca for Mg, Mn, Sr, and Se; removal/addition of Ca) were simulated, and the change in the work function and potential profile were calculated. The analysis of the obtained results with known data yields a good agreement.

Key words: hydroxyapatite, density functional theory, surface, work function, electrostatic potential, substitutions, cations, defects