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How do we perform a calculation assuming an initial anti-ferromagnetic state for a metal in bcc structure having just one atom per unit cell and an odd number of electrons? I would like to perform a total energy calculation for a bcc Fe as a function of the lattice constant assuming different spin states and then compare different curves to decide on the spin state.
By performing a spin-polarized calculation are we assuming a ferromagnetic state?
I would appreciate your reply.

First of all, if you want to simulate an anti-ferromagnetic (AFM) state you have to have a sufficiently large simulation cell. For bcc Fe this means you have to have at least two atoms. If you want more complicated AFM structures you have to have an even larger cell.

You can then initialize the magnetic moments by specifying the MAGMOM tag (provided you have set ISPIN = 2). If there is a close-by local minima for the AFM solution (as there is for bcc Fe provided you have given large enough initial values in MAGMOM) then the calculation will converge to the AFM state. Please notice that if you compress the bcc Fe phase to much (to around 9 Angstroms^3/atom) the AFM phase becomes non-magnetic.

Also, a final note: I don't know what purpose your investigation of different mag. states will have but please just notice that are a lot of previous work on this aspect by using different functionals. These references are just a selection:

Fe phases: LDA vs GGA:
Phys. Rev. B 46, 13599–13602 (1992)

Spin spirals in fcc iron (includes comments on bcc Fe as well)
Phys. Rev. B 62, 5564–5569 (2000)
Phys. Rev. B 66, 224409 (2002)

Fe phases: LDA, PBE, AM05
Phys. Rev. B 79, 155101 (2009)

Cheers,
/Dan


<span class='smallblacktext'>[ Edited Sat Apr 17 2010, 07:34PM ]</span>

Thank you Dan for your quick and very instructive response to my question. I appreciate it.