The question is simple: should I include magnetism in formation energy calculation (DFT) when I know that system is magnetic? If so, how to do that?
Magnetism affects many of parameters (in comparison with non-magnetic structure) often changing structural parameters (e.g. cell volume) and also affecting total energy.
The usual way to calculate formation energy (in non magnetic case) is to subtract energy of individual components from total energy of whole structure. Therefore I'm asking is magnetism obligatory when calculating formation energy? If yes, how to set magnetism of individual ingredients when doing SCF calculation?
So, for the structure I should make spin-polarized calculation and for individual atom simply non-polarized?
The whole picture is that I'm looking for most stable (in terms of total energy) configuration while substituting one atom at different sites. System is in general magnetic (FM) and this substitution heavily changes magnetism.
Thank You all for your kindly discussion. For now, I say problem solved.
> So [...] for individual atom simply non-polarized?
actually, that depends on what your final reference state is. if it is free atoms in a vapor (this corresponds to the heat of vaporization, usually called the cohesive energy) you must calculate the spin-polarized atoms (better in an asymmetric supercell). atomic spinpol can change your cohesive energy per atom by several tenths of an eV or more depending on the species involved.
on the other hand, you may ask what happens if your solid was broken up into, or being constructed from, molecules or other solids. for example, you might ask if Cr2O3 forms out of Cr metal and O2 molecules. you calculate E1=(energy of Cr2O3) and E2=[2 x energy of Cr metal + 3/2 x energy of O_2 molecule]. the energy difference dH=E1-E2 between the condensed phase (Cr2O3) and the reservoirs (Cr metal and O_2) is the enthalpy of formation. if it is negative Cr2O3 forms out of Cr metal and O2 (it really does). And surprise, all the compounds or molecules species involved here are magnetic (O2 being the only triplet dimer).
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