Calculating stress involves performing a stress tensor calculation, even when the cell parameters are fixed. The stress is a useful measure of the accuracy of the structural description: for example, after performing geometry optimization with a fixed cell, you can get an idea of how accurate the cell parameters are from the magnitude of the remaining stress. Similarly, when studying an isolated molecule in a supercell, the value of the stress provides a guide as to the size of the cell required to remove unphysical interactions between periodic images.
It is important to use finite basis set correction when calculating the stress tensor (see Setting up the basis set for details). Choosing either Smart or Always as the Apply finite basis set correction setting on the Basis tab of the CASTEP Electronic Options dialog will achieve this.
To perform a stress analysis
CASTEP Calculation dialog