Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
Geometry Optimisation with CASTEP
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Next: References Up: <strong>Geometry</strong> <strong>Optimisation</strong> <strong>with</strong> <strong>CASTEP</strong> Previous: A Variable Cell calculation<br />
Example - A Bulk Terminated Silicon<br />
Surface<br />
When the supercell does not have full 3D periodicity the calculation must be converged <strong>with</strong> respect to<br />
other supercell parameters. Consider a surface. We will use the Si(100) surface reconstruction as an<br />
example. Figure 5 below shows the bulk terminated Si(100) surface.<br />
Figure 5:The bulk terminated Si(100) surface<br />
In one direction we have a defect; after some point there will be a complete absence of atoms, essentially<br />
out to infinity. In the opposite direction we move into the bulk and in principle we should include an<br />
infinite number of bulk layers below the top few surface layers.<br />
Both of these requirements are computationally impractical. In practice we must include just enough bulk<br />
layers so that the inner bulk atoms remain in place during the calculation and only the top surface layers<br />
reconstruct.Also, only a finite amount of vacuum space above the top surface layer is practical. However,<br />
when a supercell is repeated periodically in all three spatial directions the bottom bulk-like region of one<br />
supercell borders upon the top vacuum region of the supercell below it (figure 6). If the vacuum region is<br />
not thick enough the uncompensated charges on the bottom of the one supercell and the top of the<br />
supercell under it will cause an unphysical interaction. Vertically adjacent supercells must not be able to<br />
'see' one another.