Tutorials: GeometryOptimisation

Computational Details

Exercise 1 - H2 dimer

Build H2.cell and H2.param files to compute the bond length of an H2 molecule.

Remember you can use castep --help to assist you with finding the appropriate input parameters.

So far you've used the local density approximation (LDA) for the exchange-correlation functional in this exercise. Repeat your calculation with the PBE exchange-correlation functional (a popular GGA):

How does this change your results?

The experimental H2 bond length is about 0.74 Angstroms and the vibrational frequency is about 4395 1/cm.

Exercise 2

Run a geometry optimisation on silicon.

You can use a silicon input file from one of the previous tutorials as a starting point for your input files.

Set CASTEP's parameters to perform a geometry optimisation using a 160 eV plane-wave cut-off energy and an 8x8x8 Monkhorst-Pack k-point grid:

Because you're going to change the lattice vectors, CASTEP will do a finite basis-set correction (FBSC); this will calculate and print out dEtotal/dlog(Ecut) anything more than 0.1 eV/atom is big and a sign of incomplete convergence. What is the final lattice parameter?

Do convergence tests (cut-off-energy, kpoints etc). The experimental lattice constant is 3.84 Angstrom - how does your value compare? What is the difference in your results between calculations using an LDA and a PBE exchange-correlation functional?

Exercise 3 - Graphite

Perform geometry optimisations on graphite using LDA and PBE. Again, you can use a previous input file as a starting point.

Exercise 4 - Extensions

Experiment with the above structures, and see how you can improve the convergence rate by tweaking the various parameters discussed in lectures. You might also want to experiment with ionic constraints and/or alternative minimisers to BFGS and/or external pressure.

Try some systems you are interested in! Play with the functionality and see what you can compute.

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Page last modified on August 18, 2015, at 02:33 PM