Dear Izaak You (often) suppose to investigate an "isolated" dopant or defect, but in fact you are always simulating an array (a superlattice) of impurities in a semiconductor (SiC) host. In principle you should verify that your results are reasonably insensitive to the supercell volume. In such a framework you can quite safely fix the equilibrium lattice parameters of the 2x2x2 bulk SiC and relax all the atoms, but if you want to study modifications to the lattice parameters induced by defects, which may depend on defect concentration, codoping, etc. You may be interested in some details: I've often investigated defects in zincblend GaAs by using cubic 64-atom supercells (and a 4x4x4 k-points mesh) at fixed lattice parameters. A 64-atoms simple cubic SiC supercell may represent a good starting point to study defects (point defects...) in SiC. You may want to read the following Zunger's paper, which (as usual...:-)) addresses some of your questions better than me.
S. Lany and A. Zunger Phys. Rev. B 78, 235104 (2008) HTH Giuseppe> Hello everyone, > > I am trying to study impurities in SiC bulk material. For this, I use a big > unitcell of 2x2x2. I do not understand how to do a relaxation in this system, > ie, which atoms should be kept fixed? I was thinking that the > unitcell boundary > atoms would automatically be kept fixed but that does not seem to be > the case. > Any help will be much appreciated. > > -- > Izaak Williamson > Research Assistant > Physics Department > Boise State University GiuseppeMattioli ISM-CNR Italy