Boundary Conditions

Boundary conditions in CASTEP are periodic in all three dimensions. However it is possible to simulate the dynamics of isolated clusters/molecules, provided that the size of the super-cell is sufficiently large that no atom interactions cross a cell boundary.

Figure 2.1: Isolated pentane molecule simulated by converging the total energy with respect to the size of the super-cell.

This involves careful convergence of the forces with respect to the size of the super-cell. This convergence should be especially strict for MD calculations as expansion of the cluster may occur, causing it reach a state where it can interact with itself, even if it did not in the initial cell. Variable cell ensembles should not be used for isolated systems.

Slab and wire boundary conditions can also be simulated by increasing the size of the super-cell in one or two dimensions only. Again convergence with respect to these dimensions is critical. Variable cell-shape simulations are possible in these cases, provided the cell is constrained in the non-periodic directions. Alternatively, an isotropic variable cell simulation can be conducted using an orthogonal super-cell for which interactions across one or more of the three cell directions have been eliminated by the addition of a vacuum gap. This is identically equivalent to application of pressure in the remaining periodic directions only.

Figure 2.2: Slab boundary conditions, simulated with a vacuum gap in the vertical direction, used to simulate bulk terminated silicon. Bonds into the bulk are satisfied by hydrogen passivation.