Developing semi-empirical ab initio based potentials in materials modeling

Ab initio calculation based on density function theory (DFT) is an accurate and efficient method for modelling material properties. It is performed by solving the Shrödinger equations with a few assumptions to obtain the physical properties of the system. It is very computational demanding when dealing with large systems or longtime simulations. Developing empirical potentials on the basis of ab initio calculations on smaller systems is a possible way to solve this problem. The empirical potentials will benefit from the accuracy of ab initio simulations and can facilitate applications to large systems and long-time simulations. In this thesis, we have performed two studies regarding fitting empirical potentials: one is fitting an empirical Sutton-Chen potential based on ab initio simulations for iron under extreme conditions and the other one is fitting an improved Finnis-Sinclair potential for ternary V-Ti-Cr alloy. In the first part, we focus on fitting a Sutton-Chen potential for solid Fe under the Earth’s inner core condition. Based on ab initio molecular dynamics (MD) simulation results, the Sutton-Chen potential is fitted to energies of the configurations obtained from ab initio MD simulations at the pressure of 360 GPa and temperature of 6000 K. The method applied for the fitting is the Particle Swarm Optimization (PSO) algorithm. The Sutton-Chen potential can reproduce the ab initio energies with an error of 6.2 meV/atom. Set as the same with ab initio MD simulations, classical MD using Sutton-Chen potential can obtain the consistent results with those from ab initio MD simulations at the pressure of 360 GPa and temperature of 6000 K. In order to explore the size effect on the results, we extend the classical MD to large-size systems (from 1024 atoms to 65536 atoms). We also extend the temperature range to see the temperature effect on the results. In the second part, we develop an improved Finnis-Sinclair (IFS) potential for ternary V-Ti-Cr alloys. The interaction parameters of V-V, Ti-Ti and Cr-Cr are fitted to the experimental lattice constants, cohesive energies and elastic constants. The binary alloy potential parameters are obtained by constructing 3 binary alloy models (V15Ti, V15Cr, V8Ti8) and fitting to their theoretical lattice constants, cohesive energies and elastic constants. Finally, the IFS potential is successfully used to calculate mechanical properties and the monovacancy formation energy in V-Ti-Cr alloy. It is also applied to investigate the composition effect on the mechanical properties of ternary V-Ti-Cr alloys.