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magnetic system giving non-magnetic solution

Elk Users
2013-03-22
2013-06-11
  • Souvik Paul
    Souvik Paul
    2013-03-22

    Hi,

                         I want a comparison between the energetics of Mn2NiSn in the cubic inverse Heusler and Heusler phases. I had previously done these comparison using a pseudopotential code (Quantun Espresso). For the inverse Heusler phase, the code (ELK) works fine. But, I get problems in calculating energies and magnetism in the Heusler structure. The Heusler unit cell has four atoms at (000), (0.5,0.5,0.5) and (0.25,0.25,0.25), (0.75,0.75,0.75). I had varied the lattice constants from 10.70 a.u. to 11.60. Up to 11.20 a.u. the scf solutions reached to a nearly zero individual and total magnetic moments state, while in the later cases, the results were as expected (~5 mu_B). The energies varies in a zig-zag fashion as a function of lattice parameters making me unable to locate the equilibrium lattice constant. I've used the same input file throughout the calculations. Please have a look at my input file below and tell me where I've done wrong……..

    tasks
      0

    epspot
      1.0E-7

    spinpol
    .true.

    gmaxvr
      14.0

    rgkmax
      9.0

    lmaxapw
      10

    lmaxmat
      6

    stype
      1

    swidth
      0.005

    xctype
      3

    maxscl
      500

    beta0
      0.05

    betamax
      0.5

    ! the large magnetic local field on Ni is halved at every iteration
    reducebf
      0.5

    ! fairly large number of empty states required for magnetic cases
    nempty
      60

    ! lattice vectors
    avec
      0.0 0.5 0.5
      0.5 0.0 0.5
      0.5 0.5 0.0

    scale
      11.00

    sppath
      './'

    atoms
      3                                                                                : nspecies
      'Mn.in'                                                                       : spfname
      2                                                                                : natoms
      0.00  0.00  0.00    0.0  0.0   2.000                        : atposl, bfcmt
    -0.50  0.50  0.50    0.0  0.0   2.000                        : atposl, bfcmt
      'Ni.in'                                                                         : spfname
      1                                                                                 : natoms
      0.25  0.25  0.25    0.0  0.0   0.300                         : atposl, bfcmt
      'Sn.in'                                                                         : spfname
      1                                                                                 : natoms
    -0.25 -0.25 -0.25    0.0  0.0   0.000                       : atpos , bfcmt

    ngridk
      12  12  12

    vkloff
      0.5 0.5 0.5

    regards,
    Souvik

     
  • Dear Souvik,

    your general setup looks OK, but I think you have got the structure wrong. Elk uses relative coordinates in terms of the lattice vectors for the atomic positions not Cartesian. Hence your -0.5 0.5 0.5 (relative)  -> 0.5 0 0, while 0.5 0.5 0.5 -> 0.5 0.5 0.5 also in Cartesian coordinates. You want the latter, right?

    The wrong structure could have lead to strange neighbour distances and problem with semi-core states which would explain your unstable energies.

    I hope this helps, good luck!
          Lars

     
  • Sorry my bad.
    0.5 0 0 and 0.5 0.5 0.5 are of course equivalent for the fcc lattice.

    Then I would suggest that you start with your non-magnetic solutions and put on the magnetic field again, i.e. essentially just put task=1 and see whether you do not converge to a magnetic solution.

    Best,
        Lars

     
  • asubedi
    asubedi
    2013-03-25

    Dear Souvik,

    The moments sometimes vanish when the external magnetic field is zero. With the value of reducebf that you are using, maybe the solution does not converge to a magnetic one. I generally start from a large external field and, once a magnetic solution is found, reduce the external field manually until the moments and total energy do not change much.

    Best,
    Alaska