In most cases the calculation of elastic constants is proceeded by full geometry optimization, including cell optimization. This means that elastic constants will be generated for the theoretical lattice constants. However, it is possible to skip this step and generate elastic constants for the experimental structure.
Elastic constants are evaluated by calculating the stress tensor for a number of distorted structures. Internal coordinates are optimized in each run, while keeping the lattice parameters fixed. The accuracy of the elastic constants depends to a great extent on the accuracy of the SCF part and also on the level of convergence of geometry optimizations for each distorted structure.
CASTEP generates the strain patterns automatically, based on the lattice type of the structure. The maximum number of patterns required is six for a monoclinic structure, while one pattern is sufficient for cubic cells. Certain symmetries allow you to modify the patterns so that whenever possible, volume-conserving strains are generated.
The quality of the calculated values depends strongly on the amplitude of the applied distortions and the number of points that are used for each strain pattern. You should use at least four steps for each strain to obtain a statistically reliable linear fit of the stress-strain relationship. The maximum strain amplitude should be chosen so as to be (i) sufficiently low that the material stays within the linear elasticity regime, and (ii) sufficiently high that the distortion generates stresses significantly higher than the error level of the CASTEP calculation (errors due to imperfect SCF convergence, for example).
To set up strain patterns
The accuracy of the calculation is controlled by the Quality setting on the Minimizer tab of the CASTEP Geometry Optimization dialog.
For a detailed description of the quality settings please refer to the Setting the quality of a calculation topic.
To change the convergence tolerances
For trigonal symmetry, the orientation standard where C lies along the z-axis and A is in the xz plane must be used. For further details on the lattice orientation standard and how to set it, see the Viewing or changing the lattice parameters topic.
Elastic constants theory
CASTEP Elastic Constants task
CASTEP Geometry Optimization task
Calculating elastic constants