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

[Katalin Gaal-Nagy]

Dear Yuehua,

it is true that the ecut gives you the number of plane waves used in the 
calculation and a test of ecut (fixed lattice, various values of ecut) 
gives you the resolution of the energy in your calculation. More plane 
waves (larger ecut) lowers the energy of your system. The variation of the 
number of plane waves with a change of the volume at fixed ecut was 
discussed in PRB 52, 8160 (1995). There, also some corrections to the 
total energy with respect to the change of the number of plane waves are 
suggested.  Of course, if you have a calculation which is converged with 
respect to ecut, these errors are very small.

So, in order to relax your system you first should test the convergence 
with respect to ecut (and the number of k) and with these values you 
should relax the system with respect to c/a and the volume of the unit 
cell. I suggest to relax the c/a for a fixed volume in order to avoid the 
spurious variations with respect to the number of plane waves by looking 
at Etot(c/a). This you should do for every volume you consider. Having the 
volumes with the correct c/a fixed, you can look at Etot(V) in order to 
get the relaxed system.

About the influence of ecutrho I cannot say anything, since it never 
played a significant role in my calculations. But maybe someone else could 
give further informations.

Good luck,
Katalin


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, >>>>>>>             Katalin Gaal-Nagy              <<<<<<< ,
,                      Phys.Dept./INFM                       ,
,                    University of Milan                     ,
,                      Via Celoria 16                        ,
,                     +39 02 50317377                        ,
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> 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:
> Basically, as you enalarge your unit cell, your brillouin zone shrinks,
> 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 .
>
> i need your help .
> thanks in advance
> 2007/12/3, Hande Ustunel <hande at newton.physics.metu.edu.tr>:
>>