[Wien] The calculated-error of the magnetic moment

[Peter Blaha]

> I had done a calculation for a cuprate, which has 40 atoms in a unit cell. 
> 
> Since our result shows this cuprate is insulator, the spin magnetic moment MUST be an integer number of Bohr magnetons. However, our calculated spin magnetic moment in unit cell is 1.01 Bohr magnetons, not equal exactly to 1.00 Bohr magnetons.
> 
> Do you think this small difference (smaller than one percent) is usual and acceptable?

Usually the (undoped) cuprates come out as NON-MAGNETIC METALS in LDA !
in contrast to experiment. Of course it could be, that your cuprate is
different, but it would be some surprise. Usually LDA+U is needed.

Typically, the Cu moments are around 0.5-0.7 uB and cuprates order
antiferromagnetic !, i.e. the total moment per cell is zero!
The small moment (1 uB /cell) indicates that the calculations are 
probably not really converged. Magnetism disappears sometimes VERY slowly
(maybe at rates of 0.001 uB / scf iteration) AND often a quite good
k-mesh + RKMax is needed!

To find out whether you have a metal or an insulator, plot the spin-up/dn
DOS or check the band-limits in case.output2up/dn.

Be careful!

Regards

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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