Free energy difference calculations comparing fcc and hcp structures using molecular dynamics computer simulations

Molecular dynamic computer simulation methods have been employed to calculate the free energy difference between the fcc and hcp structures using a Morse potential parametrized to model Ni. The calculation is carried out for a temperature (∼1000 K) where anharmonic effects are important and also for a range of interactions about each particle. In addition, at the same temperature, another calculation using only nearest neighbor interactions was performed. Using lattice sums the potential energy for the two static lattices was calculated. For the Morse potential used, static lattice sums give a lower potential for the hcp structure. With only nearest neighbor interactions, the fcc–hcp free energy difference is determined only by the entropy difference. In this case the fcc structure has the lower free energy. When both anharmonic effects and longer range interactions are considered we find, at the temperature and density investigated, that the hcp structure is more stable.