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

Elena Cannuccia elena.cannuccia at roma2.infn.it
Tue Dec 4 15:23:40 CET 2007

Sorry for my previous mail...
I didn't want to send it to everyone....
Sorry again...

> 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
> ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
> , >>>>>>>             Katalin Gaal-Nagy              <<<<<<< ,
> ,                      Phys.Dept./INFM                       ,
> ,                    University of Milan                     ,
> ,                      Via Celoria 16                        ,
> ,                     +39 02 50317377                        ,
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> , >>>>>>> katalin.gaal-nagy at physik.uni-regensburg.de <<<<<<< ,
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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>:
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