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I was trying to find an equlibrium crystal structure by a grid of calculations varying lattice parameters a, b and c(orthorhombic cell) each calculation being an ISIF=0 caclulation.(fixed cell shape and volume)

The lattice basis consists of some ions at general positions(x,y,z) and others at the special positions eg. (1/2 1/2 1/2) etc.

Is it necessary to relax each ionic position or would it be acceptible to keep the special positions fixed by selective dynamics.(that would save a lot of time too)

that depends: if you give the atomic positions in direct coordinates and if there is strong experimental evidence that some of the atoms are in high-symmerty positions, you can leave these atoms fixed if you use ISIF=0.
In any case, I would do one test run for the converged cell to see whether these atoms change their positions if they are not restricted anymore.

thanks admin for you answer, but with ISIF=0 the ions will be relaxed, I do not want to relax them, I want to see the band structure for the experimental results. for this I choose to use a selective dynamics with (FFF) for all atoms, together with ISIF=2 and IBRION=2 also. What do you think about this? or have I to set IBRION=5 or 6 only?