Ionic Relaxation With Variable Volume But Fixed Shape

[DarkSorrow]

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

[admin]

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

[zj]

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