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Hello everyone,
I'm running some pressure evolution simulations for a bunch of H2S crystal structures. My issue is that by using ISIF8 (and then ISIF2 for finer forces optimization) I preserve the space group of the structure, however the proportions between the lattice parameters are strictly maintained. Conversely, by using ISIF3 (and then ISIF2) I allow the ratios between lattice parameters to vary, however the space group is not a priori maintained.
Does anybody have a suggestion about how to preserve the space group while letting the proportions between lattice parameters vary anyway?

Hello,

Thanks for reaching out. When you say that you want to preserve the space group of the crystal structure, do you mean that the Bravais lattice type should not change during the calculation? This is not possible with ISIF=3, as it was specifically designed to allow changes in all degrees of freedom. You might still end up with the same symmetry, but this is not generally true.

If I'm interpreting your question correctly, and what you want is a very specific change in the lattice geometry that preserves certain symmetries, then you will most likely have to perform the relaxation manually. Introduce small systematic changes in the lattice parameters by editing the POSCAR file and see how the total energy changes. This should allow you to reach a minimum by repeating the process and looking for the minimum of energy.

Let me know if I was able to answer your question.

Hello,
thanks for your answer. Yes, that's exactly what I'm looking for.
So, do you suggest something like a simulated annealing?
Thank you again

I'm happy to help. While simulated annealing would work, it's not really well suited for this kind of problem. The dependence of the total energy on the lattice parameters should be pretty smooth if you are already close to the minimum. Therefore, you can just employ a simple iterative scheme or even a manual search to scan for the optimal lattice parameters while you impose your symmetry constraints.