[Pw_forum] Supercell- gamma point calculation reliability
Nektarios Lathiotakis
lathiot at physik.fu-berlin.de
Mon Sep 18 18:38:48 CEST 2006
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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