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There is a small error on line 565 in the poscar.F file in vasp.4.6.28. The POSCAR data is being written to the unit number IU, so the line

IF (LSDYN) WRITE(13,'(A18)') 'Selective dynamics'

should be

IF (LSDYN) WRITE(IU,'(A18)') 'Selective dynamics'

This only affects writing of poscar data to a file other than the CONTCAR.

Dear Graeme, thank you for that hint!
actually, the 'selective dynamics' line is of relevance for the relaxation behaviour only if read from the CONTCAR file in follow-ups of a vasp-run.
Also, the read statements reading that part written in OUTPOS to all other files besides CONTCAR (eg. in CHGCAR) expect exactly 7 lines before the positions of the atoms are read (1xheader, 4xskipping:(1xlattice constant scaling factor + 3xBravais matrix: doloop 1,4), 1x#of ions, 1x skipping the line saying either 'direct' or 'carthesian'). There is no information passed to these subroutines (eg READCHG) whether 'selective dynamics' is chosen (and hence there would be 8 lines to read) or not.
Thats the reason why 13 (CONTCAR) is explicitely given (instead of IU) in that special write statement.

Ah yes, I see what you mean - thank you for the explanation. I'll make sure to use unit 13 for any for any CONTCAR-like files.