The period of this simulation must be sufficient to obtain good statistical averages of the required properties. For many systems, the number of terms can be limited because the short range nature of the Lennard-Jones potential means that reasonable accuracy can be obtained by considering only atom pairs with interatomic distances within a cutoff range, which is of the order of a few atomic diameters. Long range corrections are used in the calculation of thermodynamic properties. For small systems this strategy is sufficient to ensure that the forces are evaluated efficiently. An algorithm has been developed by Hockney et al, the linked-list algorithm, which minimises the time taken to find atomic pairs that contribute significantly to the potential.
Cubic cells are the most appropriate for large bulk systems.
At each timestep each atom is allocated to a cell, and one atom from each cell is selected as the first atom in that cell. Provided that the cell size is chosen properly, the interatomic distances need only be calculated for atoms in the current and nearest neighbour cells. Note however that for edge and corner cells, those at the opposite edges and corners are involved because of the periodic boundary conditions. Implementation of this algorithm on a parallel machine involves the allocation of a number of adjacent cells to one processor. These are read from the standard input on channel 5.