Recipe

The essence of the calculation is for the ions and electrons in the supercell to be moved around stepwise until the forces on the atoms and the change in total energy between steps fall below some predefined convergence tolerance. The ionic positions are optimised using quasi-Newton methods. For each configuration of the ionic positions the electronic configuration is optimised using the method of conjugated gradients. The flow of the calculation is thus;

1. Move ions into new positions using geometry optimisation algorithm
2. Optimise electronic configuration using conjugated gradients method
3. Compare total energy with previous configurations and check if forces within tolerance limits
4. If structure not optimised start at (1) and generate new set of ionic positions

This cycle is performed until the forces fall within the tolerance limit and the energy should then be a local minimum.

What types of system can this be applied to? In theory CASTEP can geometry optimise most bulk systems in a wide range of materials such as semiconductors, ceramics, metals, minerals and zeolites. It can also look at defects such as surfaces. The main restriction is the amount of computational resources available to the user. Calculations scale with the cube of the number of atoms included. More about this in the discussion of supercells.