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Hi,

Apologies if this is a duplicate post, but I wasn't able to spot this elsewhere on the forum.

I am looking to optimise a structure by relaxing the ions and changing the cell volume, if necessary, but I want to keep the shape of the cell (or, at least, the 90 degree angle between the lattice vectors - my cell is orthorhombic) fixed.

Of the various ISIF tag options, there doesn't seem to be any one which will let me specify this.

The only thing I can think of is to iteratively run VASP with ISIF = 7 (no ionic relaxation, change volume, maintain shape) and ISIF = 2 (ionic relaxation, no changes to cell). While in principle I have no problem doing this (easy enough in Python), it seems like a bit of a hack.

Does anyone know whether there is a proper way to do this using the VASP options? I know it can be done with CASTEP, but its geometry optimisation options are a lot more involved than VASP's, so maybe this is a feature that has yet to be implemented.

Thanks,

Jonathan

Der Jonathan, you are correct, with ISIF it is not possible to relax the cell and the ions under the constraint that the cell shape is kept fixed

I think you can take a look about the constr_cell_relax.F.