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Dear friends,

I want to relax the graphite which has hcp structure. I have constructed a unit cell of 72 atoms (3*3*4) and have employed GGA-91 pseudopotential. But, it seems that there is fluctuation in energy in ionic relaxation. Maybe, this kind of pseudopotential does not work. Please help, if any one has an idea.

Best,
Farzaneh

as the interlayer distance between the graphite layers is governed by van-der Waals interactions, a simple DFT calculation will most probably not converge (or give unreasonable results)
van-der Waals forces are implemented in vasp.5. only.

Thank you for your comment. We have vasp 4.6. Can we update it? Do you know the update condition? Is it free?