9. The final step in creating the necessary information for the VASP code is to use Au. Similarly to the water molecule (see the H2O tutorial), you will be asked to enter the name of the original MgO geometry file CONTCAR_MgO, and then a small info will be displayed. The 1st stage is now complete.

There are 4 irreps, each has 6 vectors in it, 6x4=24 degrees of freedom as required (8 atoms, 3 degree of freedom each). This is to be expected as there are exactly 6 phonon branches in a crystal with the unit cell containing 2 atoms: 3 acoustic and 3 optical ones (again, see, e.g. my book).

Now you should find in the current directory:

1. (i)the script run_DFT_script. In it you will have to replace the line with “vasp” with whatever you use on your computer to call this code.
   

2. (ii) a set of directories containing all the input files with distorted geometries:

Each such directory contains 6 input files for the 8-atom cell, e.g.:

8. Create all the necessary group-theoretical information by Ir, below:

10. Do not forget to edit the INCAR file as well so that VASP would only run a single point calculation. Now you should run the script and wait. This will complete the 2nd stage.

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