Computational method for aperiodic systems: Molecular mechanics simulations

Abstract

For aperiodic systems with long-range interactions, an algorithm of molecular mechanics (MM) calculations is proposed. In order to treat the aperiodic system as an infinite system containing completely isolated disorder, the crystal structure is described based on the deviation from a periodic system, and potential energy and forces are evaluated as differences from those for the periodic system. The MM calculations for ionic systems containing local impurity atoms have been carried out as test calculations. The obtained lattice relaxation and energy of impurities have been compared with the results of a conventional supercell method performed based on five different sizes of unit cells, and the advantages of the proposed algorithm in terms of computational accuracy and time have been ascertained. For aperiodic systems where the charge neutrality has been lost because of the existence of ionized impurities, this algorithm realizes MM calculations without artificial charge neutral conditions which are usually required in the supercell method. The algorithm also has a possibility of the application to molecular dynamics calculations.