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
