[Pw_forum] Noncollinear magnetization and spin orbit coupling problem on Energy convergence and Spin

[Xiao Shi]

Dear PW users

I'm doing calculations on Bi2Se3 system and get some problem with the magnetization of this system.
As Bi2Se3 is a topological insulator, I need large spin orbit coupling effect which means I need to do noncollinear calculation.

After adding O2 molecule on the surface, I did spin polarization calculation where the energy doesn't converge and the system goes into anti-ferromagnetic like shown below:


     total energy              =   -5045.36386241 Ry
     Harris-Foulkes estimate   =   -5045.36391863 Ry
     estimated scf accuracy    <       0.00001751 Ry

     total magnetization       =    -0.00     0.00     2.00 Bohr mag/cell
     absolute magnetization    =     3.42 Bohr mag/cell


If I continue for more steps, it will grows into very large anti-ferromagnetic like 20 Bohr mag/cell for absolute magnetization which is unphysical. The O2 I placed is far away from the surface and there shouldn't be any spin polarization for pristine surface in reality. Also the total energy doesn't change monotonically.


The input file is listed below:
This is a supercell system of Bi2Se3 with 2*2 in the hcp lattice and there are 15 layers of atom for the slab with vacuum larger than 20ans (O2 is at least 10ans away from the slab)

 &system
   ibrav=4,
   celldm(1)=15.992,
   celldm(3)=6.555356882,
   nat=62,
   ntyp=3,
   ecutwfc=30.0,
   ecutrho=300.0,
   occupations='smearing',
   degauss=0.002,
   noncolin=.true.,
   lspinorb=.true.,
   starting_magnetization(1)=-0.001,
   starting_magnetization(2)=0.001,
   starting_magnetization(3)=0.5,
   london=.true.
   starting_spin_angle=.true.
   nosym=.true.
 /
 &electrons
  mixing_beta=0.2

ATOMIC_SPECIES
 Bi 208.98 Bi.rel-pbe-dn-rrkjus.UPF
 Se 78.96 Se.rel-pbe-n-rrkjus_psl.0.2.UPF
 O 16.00 O.rel-pbe-n-rrkjus_psl.0.1.UPF
ATOMIC_POSITIONS {crystal}
O       0.582808125     0.518307602     0.030071968
O       0.438796581     0.511316952     0.033700923
Se      0.333363441     0.666726883     0.270506612
Se      0.333363312     0.166681597     0.270506668
Se      0.833409077     0.166681597     0.270506668
Se      0.833408961     0.666727129     0.270506668
Bi      4E-16   3E-16   0.298539365
Bi      8.34671E-08     0.500045262     0.298539398
Bi      0.500045295     -2.00018E-07    0.298539398
Bi      0.500045179     0.500045262     0.298539398
Se      0.666727117     0.333363441     0.332477455
Se      0.166681746     0.333363493     0.332477478
Se      0.166681629     0.833408578     0.332477478
Se      0.666726948     0.833408578     0.332477478
Bi      0.333363498     0.166681691     0.366247219
Bi      0.833408983     0.166681691     0.366247219
Bi      0.833408867     0.666726943     0.366247219
Bi      0.333363441     0.666726883     0.366247266
Se      3.44477E-08     0.500045237     0.394371017
Se      0.50004532      -1.51013E-07    0.394371017
Se      0.500045204     0.500045237     0.394371017
Se      5E-16   3E-16   0.394371148
Se      0.166681762     0.333363525     0.438001919
Se      0.166681646     0.833408563     0.438001919
Se      0.666726916     0.833408563     0.438001919
Se      0.666727117     0.333363441     0.438001924
Bi      0.333363644     0.166681764     0.466101291
Bi      0.83340891      0.166681764     0.466101291
Bi      0.833408794     0.666726797     0.466101291
Bi      0.333363441     0.666726883     0.466101294
Se      5E-16   4E-16   0.5
Se      -1.1658E-07     0.500045162     0.5
Se      0.500045395     5E-16   0.5
Se      0.500045279     0.500045162     0.5
Bi      0.666727117     0.333363441     0.533898706
Bi      0.166681764     0.333363527     0.533898709
Bi      0.166681648     0.83340856      0.533898709
Bi      0.666726914     0.83340856      0.533898709
Se      0.333363441     0.666726883     0.561998076
Se      0.333363642     0.166681762     0.561998081
Se      0.833408912     0.166681762     0.561998081
Se      0.833408796     0.666726799     0.561998081
Se      7E-16   5E-16   0.605628852
Se      -2.67539E-07    0.500045087     0.605628983
Se      0.50004547      1.51013E-07     0.605628983
Se      0.500045354     0.500045087     0.605628983
Bi      0.666727117     0.333363441     0.633752734
Bi      0.166681691     0.333363381     0.633752781
Bi      0.166681575     0.833408634     0.633752781
Bi      0.66672706      0.833408634     0.633752781
Se      0.33336361      0.166681746     0.667522522
Se      0.833408928     0.166681746     0.667522522
Se      0.833408812     0.666726831     0.667522522
Se      0.333363441     0.666726883     0.667522545
Bi      -3.16627E-07    0.500045062     0.701460602
Bi      0.500045495     2.00018E-07     0.701460602
Bi      0.500045379     0.500045062     0.701460602
Bi      7E-16   6E-16   0.701460635
Se      0.166681597     0.333363195     0.729493332
Se      0.166681481     0.833408728     0.729493332
Se      0.666727246     0.833408728     0.729493332
Se      0.666727117     0.333363441     0.729493388
K_POINTS {automatic}
3 3 1 0 0 0


I wonder has any meet this problem before and can give me some suggestion on it?

PS: I've tried increase degauss to 0.05Ryd and decrease mixing_beta to 0.05 which doesn't help

Best Regard

Xiao Shi
Department of Chemistry
Princeton University




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