[Pw_forum] Supercell- gamma point calculation reliability

Nektarios Lathiotakis lathiot at physik.fu-berlin.de
Tue Sep 19 19:28:03 CEST 2006

Nicola Marzari wrote:

> Nektarios,
> 1) the "dispersion" from the magnetic impurities should be flat,
> and the difference in energy that you look at is within two
> cells that have the same sampling. That is good.
> 2) a simple test would be to still use 1 k-point, but the Baldereschi
> (PRB 1973) point instead of Gamma. For an orthorombic cell, I generally
> use 1/4 1/4 1/4 . PWSCf would automatically try to replicate it to
> recover full symmetry (i.e. would want to use all of the 2 2 2 1 1 1
> Monhorst-Pack mesh that are not related by any point-group operation),
> but using no_sym (or nosym ?) you can force the code to just use that
> single k-point. 1/4 1/4 1/4 is often more accurate than Gamma, and
> any difference you find would hint at insufficient sampling.

Thanks, that is a great idea!

> 3) degauss, generally speaking, should be larger than what you used if
> you were dealing with a metal. In your case, though, you probably use it
> just to smooth the path to selfconsistency, and at the ground state you
> still want integer occupations everywhere, correct ?

At the very end (after ion relaxation also) I would like degauss=0. I
guess if the
variation has lead to a semiconducting structure that should be OK. If the
structure has a metalic character then the oscillation effects will not
for convergence with degaus=0. But I havent tried sofar.

> 4) what I would worry is haivng very good ionic relaxations, since these
> can be different  in the FM and AFM states. Using the same cell for the
> two calculations (same cell parameters, same cutoff, same kpoints) helps
> a lot with the electronic accuracy, since in the energy difference you
> have cancellation of terms that might have needed more kpoints.

I am thinking to allow for relaxation of the first neighbohrs of the
(impurities / native deffects).

> 5) of course, if you need for some reason absolute k-point convergence,
> than you need to increase the number of k-points, but as long as you
> compare unit cells that are identical, you might easily get away with
> lower sampling. Still, gamma can be slow to converge - have a look
> at the literature on the vacancy formation energy in silicon.
>             nicola
Thank you very much for the advice

>> Hi all,
>> I want to calculate the energy difference of FM and AFM states of two
>> isolated
>> magnetic impurities in a semiconductor, optionaly with the presence of
>> native
>> impurities.
>> The supercell has to be  fairly large, of the order of 100 atoms, so I
>> can only
>> afford Gamma-point calculation. The difference in the energy of
>> different magnetic states might be of the order of mRyd.
>> My question concerns the convergence parameters and especially degauss.
>> Right now I use degauss=5.d-4 and thats the smallest I can  use without
>> serious oscillation problems. That is 0.5mRyd so it is probably fine.
>> A second concern is the realiability of the Gamma point calculation
>> itself.
>> My worry is that increasing the mumber of k-points will lower all the
>> energies
>> by much more than the mRyd differences I am obtaining.  In other words,
>> I am worrying that the error of restricting the k-space to the gamma
>> point
>> is much larger than the energy differences I get.  What is the
>> experience on that
>> issue? Is a supercell of 100 atoms big enough to guarantee that a
>> gamma-point
>> calculation will have accuracy of the order of miliRydberg? And if not
>> are the
>> energy differences of any value or it is just crap?
>> Here are the convergence parameters I use right now:
>> etot_conv_thr = 1.d-5
>> degauss = 5.d-4
>> conv_thr = 1.0d-6
>> Thanks
>> Nektarios
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