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

xu yuehua njuxuyuehua at gmail.com
Mon Dec 3 10:20:00 CET 2007

```thank hande for your advice .
but i am still puzzled below:
(1) last time ,i saw a titled " scan  the lattice "discussion ,and you could
find it in forum,they said:
and at constant cutoff it means that more lattice vectors (i.e. plane
waves) enter into the fixed cutoff sphere. More plane waves means a
systematically larger basis set (this is one of the good things
of plane waves, it's easy to make the basis set more and more complete -
a nightmare in Gaussian), and, variationally, a lower energy. You see in
fact your energy drop going to the left.

Check then what is the number of plane waves for each of your
calculations, but most likely the wfc cutoff is the culprit

so i feel if i keep the encut fixed,and enlarge the lattice ,the more number
of plane waves will enter into the calculation ,and will get the lower
energy.  i do not understand what is the relationship between more plane
waves and a lower energy .
and just now you told me to set bigger encut and ..,seems opposite  to the
above discussion .
maybe my understanding is wrong .

2007/12/3, Hande Ustunel <hande at newton.physics.metu.edu.tr>:
>
> 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
>
> --
> Hande Ustunel
> Department of Physics
> Office 439
> Middle East Technical University
> Ankara 06531, Turkey
> Tel : +90 312 210 3264
> http://www.physics.metu.edu.tr/~hande
>
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>

--
Xu Yuehua
physics Department of Nanjing university
China
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