The influence of the simulation box geometry in solid-state molecular simulations: phase behaviour of lithium iodide in a dynamic Monte Carlo simulation

In thermodynamic computer simulations, it is common to use cubic simulation boxes, which are then regarded as unit cells of an infinitely large cubic lattice. While this approach is adequate for gases and liquids at low densities, for dense liquids and solid cuboid boxes forming an orthorhombic lattice or parallelepiped boxes forming a triclinic lattice are shown to be advantageous, because they do not predetermine the structure of the simulated system. In this work, an extension of the Ewald summation formalism towards a parallelepiped lattice symmetry is given. Monte Carlo simulations of lithium iodide with cubic, cuboid and parallelepiped box geometries are reported; the latter is found to offer little improvement over the cuboid geometry. The existence of two hexagonal solid phases is discussed.