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Hi,

Can I perform ISIF=2 on a 2 dimensional unit cell like graphite. I want to consider only the first layer of graphite. I tried running the program, but I got an input/outut error 137:value not recognized. I also got an error that says The real-space-projection scheme for the treatment of the nonlocal pseudopotentials has been switched on -- but on file POTCAR I have not found any entries signalling me that you have ever performed a real-space optimization of all nonlocal projectors!

I can run 3 dimensional graphite without any problem. I changed the POSCAR file accordingly. Any suggestion would be appreciated!

Thanks!

1) it is not conflicting to use ISIF=2 for 2D unit cells. However, for your purpose it is more appropriate to use the possibility to choose 'Selective dynamics' in POSCAR and specify the atoms which are allowed to relax: after each ion position a triplet of 'T' or 'F' defines whether this ion is relaxed along the respective x,y, or z co-ordinate.
(please have a look into the online handbook,
http://cms.mpi.univie.ac.at/vasp/vasp/node63.html
on how to use the 'Selective Dynamics' feature)

2) unless turned off explicitely (LREAL=.False.), VASP switches to real space optimisation automatically if the number of atoms in the unit cell exceeds 16. in order to save computing time for large systems

I can choose Selectice dynamics to fix the coordinates of the bottom carbon atoms in graphite. But, then I want to use a unit cell with only x and y axis, without z axis, with lattice vectors and lattice constants for only x and y axis. Can I do that?

no, VASP uses 3-dimensional boxes for all unit cells by default

no, VASP uses 3-dimensional boxes for all unit cells by default. Surfaces are modelled by 3D repeated slabs (2D-slabs separated by vacuum of sufficient thickness along z)