[gpaw-users] [gpaw-developers] Questions on EquationofStates in pw mode

[Thomas Olsen]

I think you are right that the physically meaningful thing to do, is fixing the cutoff energy. But the bulk modulus and lattice parameter are much easier to obtain with a fixed set of plane waves. If the cutoff is fixed, the eos curve will make small discrete jumps at the cell sizes where the basis set changes. This is typically a rather small effect, but it becomes very annoying when one has to fit a curve the data points. It is circumvented by using a fixed set of plane waves and you just have to make sure that the cutoff is chosen large enough, such that the calculation is converged in the largest unit cell. 

The exact same thing appears in grid mode if the grid spacing is fixed instead of the number of grid points.

BR Thomas

________________________________________
Fra: gpaw-developers-bounces at listserv.fysik.dtu.dk [gpaw-developers-bounces at listserv.fysik.dtu.dk] på vegne af jun yan [junyan at stanford.edu]
Sendt: 9. januar 2013 23:00
Til: Wellendorff, Jess
Cc: gpaw-users at listserv.fysik.dtu.dk; gpaw-developers
Emne: Re: [gpaw-developers] [gpaw-users] Questions on EquationofStates in       pw mode

Yes in principle at very high cutoff energy it should yield the same results,
However, when one performs EOS calculations, generally 700 eV (maybe I am wrong about this number) cutoff is used.  Around this energy the uncertainty in the total energy can be 20-30 meV, which is not important for total energy purposes, but can be important for determining lattice parameters.
I haven't performed tests on which methods converge faster with increasing plane wave cutoff…

All the best,
Jun

On Jan 9, 2013, at 1:54 PM, Wellendorff, Jess wrote:

> I guess both methods should in principle yield the same lattice constant in the limit of very high cutoff energy.
> So which of them seem to be closest the the "fully converged" result?
> That is, which number (lattice constant) changes the most if increasing the cutoff in both methods?
>
> - Jess
>
> On Jan 9, 2013, at 1:41 PM, jun yan wrote:
>
>> Dear gpaw users/developers,
>>
>>    Please excuse me if my following question is stupid:
>>    When one performs the EOS in pw mode, the number of plane waves are forced to be the same with different volumes of cells (let's say the lattice parameters are scaled from 0.95 to 1.05) by using the command PW(ecut, cell=cell_at_scale_1.0). Using this way, the EOS fitting curve looks perfect. It however means that, with increasing cell volume, the effective plane wave cutoff energy is decreasing since the number of plane waves is fixed. In my calculations, the decreasing of plane wave cutoff energy results in increasing of the total energy. The consequence is thus the lattice parameters shift to smaller values using fixed number of plane waves compared to using fixed plane wave cutoff energy. My question is, which one is physically meaningful : fixing the number of plane waves or the effective plane wave energy cutoff ? I will think its the latter. However, the EOS based on using the same plane wave cutoff energy looks much worse than fixing the number of plane waves. I!
>> appreciate if anyone has any insights on this. Thanks !
>>
>> All the best,
>> Jun
>>
>>
>>
>>
>>
>>
>>
>> _______________________________________________
>> gpaw-users mailing list
>> gpaw-users at listserv.fysik.dtu.dk
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>
> -----------------------------------------------------------------------
> Dr. Jess Wellendorff
> Postdoctoral Researcher
> SUNCAT Center for Interface Science & Catalysis
> SLAC National Accelerator Laboratory, U.S.A.
> -----------------------------------------------------------------------
>


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