Tutorials

# Elastic Constants

## Learning Objectives

• Use Andrew Walker and Dan Wilson's python scripts for computing elastic constants.

## Introduction

Andrew Walker (building on work by Dan Wilson) provides some python scripts for calculating elastic constants using CASTEP, available from https://github.com/andreww/elastic-constants. The tool comes in two parts - first is `generate_strain.py`, this takes the results of a successful variable cell geometry optimisation and creates a set of input files to be run through CASTEP. The second part is `elastics.py`, which takes the CASTEP results and constructs the matrix of elastic constants, Young's modulus, Poisson's ratios and bulk and shear moduli. Usage information for each script can be obtained by using the `--help` option.

## Exercise

For this exercise we will use rutile titanium dioxide. Below are example `.cell` and `.param` files to start from. It is assumed that the scripts `generate_strain.py` and `elastics.py` are in your `PATH` and that python is installed.

Task 1: Perform a variable cell geometry optimisation and make sure to include `calculate_stress : true` in your `param` file. Test the kinetic energy cut-off and k-point grid such that the stress is converged to within the default `geom_stress_tol` value. (Hint: Use CASTEP's built-in help utility to find the default.)

Task 2: Use the `generate_strain.py` script to generate a set of `.cell` files deformed according to the appropriate strain pattern. The command "`generate_strain.py TiO2`" should be sufficient.

You should now have twelve (in this case) sets of input files - individual `.cell` files and corresponding `.param`, symbolically linked to the original TiO2.param file. Notice that the new `.cell` files all have the "`FIX_ALL_CELL true`" option set.

Task 3: Run CASTEP on each of the 12 sets of input files. Note that the cell distortions can break the symmetry of the crystal, hence changing the number of k-points in the symmetry reduced sample. The "`-dryrun`" option of CASTEP can be used to do a quick check for how many k-points are required. This can help when selecting how many cores to run the calculation on if you are running CASTEP in parallel.

Task 4: Run the `elastics.py` script to obtain the elastic constants. The command "`elastics.py TiO2`" will print the results to the terminal. The "`--latex`" option generates a LaTeX formatted summary of the results and the "`--graphics`" option produces a graphical representation of the stress-strain fits in a `.png` file.

Task 5: Investigate how the results and their errors change with smaller values of the `elec_energy_tol` parameter. Also investigate the effect of changing the `--strain` option to `generate_strain.py`. Move any old `*cij*.castep` files to another directory - the `elastics.py` script only checks the first set of data in a concatenated `.castep` file.

### Example Input Files

`TiO2.cell`

``` %BLOCK lattice_cart
4.594 0.000 0.000
0.000 4.594 0.000
0.000 0.000 2.959
%ENDBLOCK lattice_cart

%BLOCK positions_frac
Ti  0.000  0.000  0.000
Ti  0.500  0.500  0.500
O   0.305  0.305  0.000
O  -0.305 -0.305  0.000
O   0.805  0.195  0.500
O  -0.805 -0.195  0.500
%ENDBLOCK positions_frac

symmetry_generate

kpoint_mp_grid 3 3 3
```

`TiO2.param`

``` task             : geometryoptimisation
cutoff_energy    : 700 eV
xc_functional    : PBE
max_scf_cycles   : 100
calculate_stress : true
opt_strategy     : speed
num_dump_cycles  : 0
```