Molecular Dynamics Computer Simulation Studies of Close-Packed Solids

Close-packed (polytypic) structures, found to be metastable during studies of structural phase transformations among close-packed crystalline systems in molecular dynamics computer simulations, are considered in conjunction with the often-used periodic boundary conditions and potential cutoff range. After presenting a denotation of those polytypes compatible with periodic boundary conditions, the computation of the fractional particle coordination numbers exhibited by polytypic structures is discussed and exemplified. The limited structure recognition due to the use of a cutoff range in particle coordination number calculations is noted, and computations of geometrical structure factors as well as a graphical display scheme are used to exemplify complete structure recognition. Some significant consequences of the restraints on the possible structural results in the molecular dynamics simulation method of studying close -packed crystals should be recognized when interpreting the results of employing this method of study. Only the denotation of polytypes compatible with periodic boundary conditions together with their structure recognition are considered; transformations among these structures are not considered in this paper. The molecular dynamics computer simulation method is next utilized to calculate various thermodynamic quantities for both fcc and hcp systems interacting with a specifically parameterized central pair potential. A discussion of the determination of error in the computation of such quantities is included with the results. Order of magnitude agreement with experimental values is found for response functions computed from fluctuation formulas. Finally, the long-standing question of thermodynamic relative stability of fcc and hcp structures is investigated by a direct calculation of their free energy differences using the molecular dynamics simulation method. Making the first known use of a specifically parameterized central pair potential in such a calculation, the entropy contribution to the free energy difference is found to favor the fcc phase, in contrast to the potential energy contribution. At the temperature investigated (T (TURNEQ) 1000 K for Ni), the free energy difference is found to favor hcp, in contrast to experimental findings. The current analysis leaves unresolved however the results of such an analysis at nucleation temperatures.