[Pw_forum] a complex and old question about why the erengy is down when i enlarge one of the crystal lattice

Hande Ustunel hande at newton.physics.metu.edu.tr
Mon Dec 3 09:33:03 CET 2007

Dear Xu Yuehua,

> hi everyone:
> my question is somewhat old ,i have seen a lot of related discussion in this
> forum .but my results does not like previous instance.
> i only want to relax c of crystal lattice ,and fixed c,computes it,and then
> change it  ,compute it,and .....
> i get the  E VS C tabular

It seems like you have explored only one side of the minimum making your
plot lopsided. In order to get a good fit, you need to go to c values that
are a bit larger than 2.85. As it is your polynomial fit will be rather
unhealthy. Along the same lines, once you have gone further towards larger
c, you could discard the two smallest c, namely 2.655 and 2.725. To make
the curve smoother try a larger ecutwfc and a larger ecutrho. I would
probably go for 35 Ryd and 350 Ryd. 

> (2)and another question is although i can not get the lowest erengy,but i
> can get the parabola of  the stress. maybe someone the stress is not
> consistent with energy,for i only want to    know  which  structure is best
> ,so the exactly stress is not so import for me, i only want to know the
> trend .in this point,whether i can rely on the stress to determine the best
> structure.

I would still try to fix the energy curve. Not only does this give you the
lowest energy of the structure, a smooth curve is at the same time a good
indication that your calculations parameters such as ecutwfc are good.

Best of luck,

Hande Ustunel
Department of Physics
Office 439
Middle East Technical University
Ankara 06531, Turkey
Tel : +90 312 210 3264

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