Molecular simulation often tries to approximate the thermodynamic limit in which the systems are very large. Since this is impossible to achieve computationally, a simulation cell must be employed. This cell being finite, it would appear that it must be bounded in some way. However, finiteness does not imply boundaries, as the circle demonstrates.
Thus, periodic boundary conditions are typically employed for the simulations of bulk materials (either disordered, or crystalline, in which case the cell must be carefully chosen.) In confined systems periodicity is only required in some spacial dimensions. Sometimes non-periodic boundary conditions are nevertheless employed.
- M. P. Allen and D. J. Tildesley "Computer Simulation of Liquids", Oxford University Press (1989) (+computer codes on the CCP5 website)
- Daan Frenkel and Berend Smit "Understanding Molecular Simulation: From Algorithms to Applications", Second Edition (2002) ISBN 0-12-267351-4