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