PyCASPESA: A new method for crystal structure prediction

Predicting crystal structure of a material is one of the most important problems in computational material sciences. There are several computational methods that developed for solving crystal structure prediction (CSP) problems. In all methods that developed for this purpose, firstly CSP problem is turned into a global optimization problem and than this problem is solved by using different methodologies. CrystAl Structure PrEdiction via Simulated Annealing (CASPESA) is one of these methods and the method uses simulated annealing (SA) algorithm for solving the global optimization problem. Within the scope of this thesis, it is intended to improve CASPESA by merging simulated annealing algorithm and density functional theory (DFT) calculations. The new program which is named as PyCASPESA is written from the scratch and it is written in Python programming language. DFT calculations in this new program were performed by using GPAW under ASE and Quantum Espresso. The program was improved in many aspects. The space search algorithm of the program was refined, therefore it achieves higher success by making fewer trials in less time. PyCASPESA is more user-friendly in the matter of input file preparation. Unlike CASPESA, the new implementation can work with the systems that has no unit cell. In addition to these, PyCASPESA is better at maximizing the shrinkage of the unit cell in volume. After these improvements, the program performs more robust CSP. PyCASPESA is applied for determining the crystal structures of magnesium borohydride (Mg(BH4)2) which is a promising hydrogen store material. Mg(BH4)2 is selected as the test case, because it has experimental data that gives us the opportunity to verify the program whether is working properly or not. The known tetragonal structure of Mg(BH ) with the symmetry group I4m2, No. 119 is the true ground-state 4 2structure. The program successfully yielded the structure with the symmetry group I4m2. In this structure, each magnesium atom was surrounded by four BH4 groups. As a result of this study, crystal structures for Mg(BH4 )2 with the symmetry group F222 (No. 22), I41 22 (No. 98), Ima2 (No. 46), Fdd2 (No. 43), Ama2 (No. 40), I21 2121 (No. 24), Cc (No. 9), P1m1 (No. 6) and C2 (No. 5) is found. In addition to these structures that reported in the literature before this study, the program is also found the new structures that did not reported in the literature with the symmetry group P4n2 (No. 118), P42m (No. 111) and Cm (No. 8).