Example - Bulk Silicon

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.

Figure 1: Bulk silicon

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.

Figure 2: The primitive cell which we will take as our supercell

The complete .cell file for this supercell is included in the link.

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 optimised.

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

castep <seed>

ie

castep si2

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.