How are you applying the stress? Please could you post your input files, so we can take a look?

I apply an isotropic stress when I optimize the cell : I put for example 50 GPa in the equivalent hydrostatic pressure. The question, is it correct if I want to obtain a space group transition...or if in CASTEP there is an other way (I use Material studio 2023). Thank you

That should be OK, but an isotropic pressure won't break any symmetries. Are you breaking the cell symmetries yourself? (In fact, you might want to check that you aren't constraining the symmetry!)

Do the two structures have the same number of atoms per unit cell?

I want to obtain high-pressure CaC2 phase transitions. I want to change from CaC2 (I4mmm with 6 atoms/cell) to CaC2 (R-3m with 9 atoms/cell) and there I don’t know exactly how to do it with CASTEP (material studio). You can help me.

Thank you.

If you just want the transition pressure, you can get that from the common tangent on an energy-volume (E-V) plot of the two phases. The energy, E, should be the energy per atom, and the common tangent is dE/dV (where V is the cell volume).

If you actually want to investigate the transition process or mechanism themselves, you'll need to use a cell which *could* be in either phase, so the number of atoms will need to be a multiple of both 6 and 9, and you might need to add some noise to ensure some symmetries are broken as you try to trigger the transition.

Ok. Now I think that I must tray with your idea. Sorry for my English.

Thank you.