Geometry Optimisation with CASTEP

Figure 7:A hydrogen passivated
Si(100) supercell with a 7 Åvacuum
gap and 9 layers of silicon.
This is a crucial feature of performing geometry optimisation on surfaces. The initial surface unit cell
must be large enough to enclose any expected reconstruction. A Si(111)-1x1 supercell will not
reconstruct into the Si(111)-7x7 Takayanagi reconstruction! If no suggestions have been provided as to
possible reconstructed periodicities by previous theoretical or experimental work then the user might
have to begin with a large supercell and perform a variable cell calculation (in case of lateral relaxations).
Now we must perform the abovementioned supercell convergence checks. We will use a 9 k-point MP
grid with a cutoff energy of 260eV for this (we will converge our calculation with respect to basis set
parameters later). Let us start with a large vacuum gap of 15Å (I know that this is entirely sufficient from
experience). We will now be able to perform several calculations with 7, 8, 9, 10 and 11 layers of silicon
and not have to worry about the vacuum gap - one problem at a time. The supercell lattice matrix and the
atomic coordinate list in the .cell file for 7 layers of silicon looks like
%BLOCK LATTICE_CART
7.6801695932 0.0000000000 0.0000000000
0.0000000000 3.8801700000 0.0000000000