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I have a system composed of 20 Fe atoms in a 5-layer 2x2 stack (bcc) with the bottom three layers frozen using "F F F" after the atom's coordinate listing in the POSCAR file. However, when I look at the result, the "fixed" atoms have moved by almost 0.1 Angstrom! Further, it is not a constant translation of all atoms. Am I missing something or is there a bug in VASP that moves these atoms? I am very concerned because the system is tangibly changed by such movement.

1. Have you checked if you have added the Selective dynamics tag correctly to the POSCAR-file?

2. Check if you do relax the volume/shape of the unit cell. Since the "F F F" locks the direct coordinates the positions might change if the basis vectors change during the run.

If none of the above helps, please provide your INCAR+POSCAR files so that we can give you a more detailed response.

Cheers
/Dan

<span class='smallblacktext'>[ Edited Tue Sep 30 2008, 07:28AM ]</span>

I did not have the selective dynamics tag, so I will try it with the tag.

this explains the behaviour. If Selective dynamics is not set, the Ts and Fs are ignored.