The CASTEP Elastic Constants task allows you to run a set of CASTEP calculations which provides all of the information necessary to obtain the full 6 × 6 tensor of elastic constants for a 3D periodic structure with any symmetry.
CASTEP automatically generates the set of distorted structures that is required by symmetry to produce the full set of elastic constants, then submits jobs for the ideal structure and for all the distorted structures.
The actual calculation of elastic constants, based on the values of initial strains and calculated stresses, is performed at the Analysis stage.
The accuracy of the elastic constants, especially of the shear constants, depends strongly on the quality of the SCF calculation and in particular, on the quality of the Brillouin zone sampling and the degree of convergence of wavefunctions. Therefore, you should use the Fine setting for SCF tolerance and for k-point sampling and a Fine derived FFT grid.
It is not necessary to perform geometry optimization before calculating elastic constants, so you can generate Cij data for the experimentally observed structure. However, more consistent results are obtained if you perform full geometry optimization, including cell optimization and then calculate the elastic constants for the structure which corresponds to the theoretical ground state.
CASTEP minimizes internal coordinates for each distorted structure while keeping the lattice parameters fixed. It is possible to use either the BFGS or the damped MD geometry optimization method. To select a different method you have to select the Geometry Optimization task, change the relevant setting and then return to the Elastic Constants task.
As part of the Elastic Constants task, CASTEP performs a single point energy calculation for the undistorted structure. This allows you to analyze electronic and volumetric properties. The energy and stress from this calculation are not used to evaluate the elastic constants.
Elastic constants theory
Setting up an elastic constants calculation
CASTEP Geometry Optimization task
Calculating elastic constants