At this point we introduce geometry optimisation of bulk Silicon as an example. We will use the known bulk structure as a
starting point and check what structure CASTEP suggests and what the bond length is optimised to.
Bulk Silicon is shown in figure 1 below.
We must now choose a supercell to enclose as few atoms as possible but still reproduce the whole structure. The following
configuration can be used provided we include the relevant symmetry operations in the cell file.
The first thing that we notice is that we are using the abc format to specify the size and shape of the lattice. The
shape of our unit cell naturally suggests that we do this.
The lengths of a,b and c are the same as those in the bulk lattice. Later we will do a variable cell calculation
as both an example of how to set one up and to check how accurately the primitive cell shape and contents can be
Note that we are using a 2x2x2 Monkhurst-Pack grid and also the inclusion of symmetry_generate to ensure
that castep is aware of the symmetry operations that our supercell satisfies.
The parameters file si.param is here
Notice that we can include the parameter line
Comment : value
This will appear in the output as a title (see the next section).
Having set up our input files we can now run the job. The job is small and we will execute it on a desktop pc under the command prompt. To do this we type
when we are in the folder in which the castep executable and the input files are located (including the pseudopotential files). In the next section we will use the results of this calculation to discuss the output files.