Geometry Optimisation with CASTEP

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