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I have a system where I have fixed the positions of the ions in the x- and y-directions as well as the cell size in those directions and want to do ionic relaxation based only on the forces in the z-direction. Is it possible to specify so that EDIFFG<0 only considers the forces in the z-direction?

Regards
Anders

Use the flags T and F in the POSCAR file with each atom to decide which directions are allowed to relax. For example an atom the has the coordinates

0.000 0.000 0.000 F F T
will relax in z only but be aware the cell vector in z-direction will not change.
If you want to the cell vector in z to change , you have to recompile vasp by modifying the CELL_CONSTR_RELAX file (Read the commented out lines in this file to learn)

In Vasp, one can relax atoms along a particular vector by tagging Selective Dynamics below the Direct tag in POSCAR file. Also, you need to tag F F T [ T implies relaxation is on along the Z vector]. The point is if you want to relax along Z direction only, make sure to construct an orthogonal supercell for your system or atleast alpha=beta=90.


Regards
Biswaranjan Dash

F F T along side all the co ordinates. Take a look at this section of the POSCAR file.

mg[48atom]-GSF-[0110]-4b_12-z-relax
1.0
6.38000000 0.00000000 0.00000000
0.00000020 11.05040000 0.00000000
0.00000029 0.000000000 15.54003731
48
Selective dynamics
Direct
0.00000000 0.00000000 0.00000000 F F T
0.25000001 0.25000000 0.00000000 F F T
0.25000001 0.05555648 0.16666627 F F T
0.00000002 0.30557148 0.16666627 F F T
0.00000005 0.94444296 0.33333253 F F T
0.25000002 0.19444296 0.33333253 F F T
0.25000003 0.99999943 0.49999880 F F T
0.00000004 0.25001443 0.49999880 F F T

regards
Biswaranjan Dash