Dear Colleague, Le 28/04/2011 04:59, ?? a ?crit : > Dear, > Recently I have calculated a system in cubic crystal structure (No.225, > Fm-3m). The compound contains two magnetic ions A (0 0 0) and B (0.5 0.5 > 0.5). The experimental reported magnetic structure is each of the magnetic > moments of A and B orders in the type I arrangement while the moment of A and > B are antiparallel with each other. > My first question is: when I design the magnetic structure as the experiment > reported, the crystal structure reduced to be tetragonal (No.123, P4mmm). > Will this symmetry reduction in calculation influence the results?
No, except that you impose a given symetry to the spin density. Thus the spin density will relax in this symmetry during the SCF cycle. > Since my A ion is rare earth elements and the calculation from VASP (use the > P1 symmetry) gets no magnetic moment for it. The earlier literatures have > reported the magnetic moments of rear earth ions may be closely relative to > the local crystal filed. Therefore I wonder whether the magnetic moment for > the rare earth in my case from wien is reliable. You should differenciate the local symetry (local crystal field) and the space group. The local field around A is always the same, whatever is the space group you are using to describe the system (Fm-3m, P4mmm or P1). So the magnetic moment will not be affected by the choice of space group, except if it leads to a different magnetic order than the experimental one. Concerning your VASP calculation, to have a magnetic moment on A you need to use a adequate pseudopotential (in which the f states are in the valence) and GGA+U. It should lead to similar results than WIEN2k. > My second question is: The magnetic moment of A ions converged to be > parallel from GGA calculation (The spins have been inverted in case.inst > file). To get the antiparallel magnetic moment of A ions. I do two GGA+U > calculations. For the first one I do it based on the result from GGA > calculation. The magnetic moment of A ions from this GGA+U calculation is > sure to be parallel. For the other one I also use the GGA results. The > difference is I use the case.dmat files from the first GGA+U calculation but > invert the density matrix of the A ion whose spin has been inverted in > case.inst file. Then I use ?-orbc? option. I did get antiparallel magnetic > moment of A ions in the second calculation and the total energy is much lower > than the first GGA+U calculation (about 3 Ry/f.u.). It that OK, or any > problem? I would restart the calculation after using -orb to finalize the calculation. Just for information, did you simply try to invert the magnetic moments in the case.inst file and directly run GGA+U? Regards Xavier > Any words will be appreciated! > Best regards! > > > > ????????wangjingjing at ciac.jl.cn > ??????????2011-04-28 > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
