Hexagonal close-packing structure on a cubic cell.

The solid phases at high pressure of atomistic systems with simple interaction models usually present close-packing structures. These structures are usually described in terms of stacking of close-packed triangular lattice layers with stacking sequences ABABAB for the hexagonal-close-packed hcp structure and ABCABC for the face-centered cubic fcc . The relative thermodynamic stability at given conditions of temperature and pressure of these two structures depends on the specific details of the pair potentials. The freeenergy difference between both phases uses to be small and it is often assumed that fcc is the stable crystalline phase; this is due to the fact that the fcc structure can be built up on a cubic cell, which makes the management of periodic boundary conditions PBCs easier in simulations, whereas in the simulation of hcp phases other geometries for the simulation cells are usually chosen. In practice, many simulation codes are restricted to work with cubic boxes. It is therefore interesting to analyze the possibility of building up a hcp structure compatible with a cubic box. Let us, firstly, remind how the fcc structure can be built in a cubic simulation box of length L. We consider systems with N=4m3, with N being the number of sites and m a positive integer number. The particle positions are set to be kxl ,kyl ,kzl , with l=L / 2m , kx, ky, and kz being integers, that fulfill 0 k 2m and kx+ky +kz being an even number. Within this realization of the fcc structure we can consider the different layers introduced above to be perpendicular to the 1, 1, 1 vector. We can then identify the layer of a given position, P x ,y ,z m , by adding up the site coordinates.