Geometry Optimisation with CASTEP

Figure 6:A finite vacuum gap may cause an
interaction between neighbouring atomic
layers in vertically adjacent supercells
Other effects are caused by uncompensated charges. The bottom layer of the supercell, which should be
bulk-like, may reconstruct. To prevent this we can constrain these atoms to remain in their bulk
positions. Another effect is an interaction between the top surface layer uncompensated charges and the
bottom layer uncompensated charges through the intermediate layers. The supercell must contain
sufficient numbers of layers of atoms to space the top and bottom layers out so as to minimise the
internal interactions in the supercell. A neat trick is to use hydrogen passivation. In this the dangling
bonds of the bottom bulk like layers are compensated by bonding these atoms to hydrogen atoms. This
introduces extra atoms into the calculation but reduces the overall number of atoms needed because the
number of spacer layers in the supercell is reduced (also, hydrogen is computationally cheap). The
thickness of the vacuum gap can also be reduced when hydrogen passivation is used (remember empty
space is a computational expense too).
The Si(100) surface is known to have a simple 2x1 dimerised structure. The small in-plane unit cell of
the reconstructed surface means that we can use a small supercell and we do not need many atoms. The
following initial supercell will be sufficient (figure 7). Notice the hydrogen passivation.