[Pw_forum] Supercell- gamma point calculation reliability

Nicola Marzari marzari at MIT.EDU
Mon Sep 18 22:17:32 CEST 2006


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.

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 ?

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.

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.


> 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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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu

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