[Pw_forum] PW relax cannot move atoms

[Li, Run]

Hello,

I have a questions about relax and vc-relax using pw.x. I tried to use them to optimize the geometry of graphite, and the atoms can barely move even though I move them manually I can get lower energy. I'm not sure why.

For example, I take 7 layers of ABAB stacked grapheme, fix the bottom 6 layers using the geometry parameters of graphite(000 thing), and relax the top layer. Geometry is defined as ibrav=14, celldm(1)=xx, celldm(3)=22.0(should be big enough), nosym=.true. Default bfgs optimization parameters are taken.  The calculation usually takes a few iterations and stops, with the top layer moved by only 0.0002 A. But if I manually pull the top layer away by 0.7A, the energy lowers by 0.0005Ry. It also didn't help by increasing trust_radius_max and trust_radius_ini to 3.0 and 1.0. Is this behavior expected? How do I make it converge to a real minimum?

I can run test calculation example02/al001.mm.in just fine.

Thank you very much,

Rain
Department of Chemistry
University of North Dakota
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