[Pw_forum] Supercell- gamma point calculation reliability

[Nektarios Lathiotakis]

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