Can we consider SOC in FM materials using QE?

Question

This is a follow-up question to my question posted here. I've been trying to study the effects of spin-orbit coupling (SOC) on the band structure of ferromagnetic (FM) material VSe$_{2}$. The input file for the SCF calculation using Quantum ESPRESSO (QE) is given below.

&CONTROL
  calculation = 'scf'
  outdir = './outdir'
  verbosity = 'high'
  tprnfor = .true.
  tstress = .true.
  pseudo_dir = '.'
/
&SYSTEM
  ibrav = 0
  A =    3.35381
  nat = 3
  ntyp = 2
  ecutwfc = 55
  ecutrho = 650
  starting_magnetization(1) = 1
  starting_magnetization(2) = 0
  lspinorb = .true.
  noncolin = .true.
  occupations = 'smearing'
  smearing = 'mv'
  degauss = 0.005d0      
/
&ELECTRONS
  conv_thr = 1e-8
  mixing_beta = 0.7d0
/
CELL_PARAMETERS {alat}
  1.000000000000000   0.000000000000000   0.000000000000000 
 -0.500000000000000   0.866025403784439   0.000000000000000 
  0.000000000000000   0.000000000000000   2.087308375869810 
ATOMIC_SPECIES
  V   50.94150  V.rel-pbe-spnl-kjpaw_psl.1.0.0.UPF
  Se   78.96000  Se.rel-pbe-dn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {crystal}
 Se   0.666666666666667   0.333333333333333   0.774935000000000 
 Se   0.333333333333333   0.666666666666667   0.225065000000000 
 V   0.000000000000000   0.000000000000000   0.000000000000000 
K_POINTS {automatic}
  6 6 6 0 0 0

This did not converge when I consider the starting_magnetization of Vanadium to be 1 but converges when I assign a starting magnetization of 0 to Vanadium.

And I couldn't find any example of SOC being considered for Magnetic systems.

So is it necessary to start with a starting_magnetization of zero for magnetic systems while considering SOC?

Answer 1

And I couldn't find any example of SOC being considered for Magnetic systems.

• There are many examples that consider the effect of SOC in magnetic systems. For example, the monolayer LaBr$_2$ in this paper, in which the author compares the band structures for three cases as follows:

• A bulk case also could be found in this paper: Phys. Rev. Lett. 122, 206401

So is it necessary to start with a starting_magnetization of zero for magnetic systems while considering SOC?

• In short, the answer is NO.

• The magnetic materials usually are calculated with spin-polarized mode (collinear), in which the electrons are classified into spin-up and spin-down and you only assign the initial magnetic momentum for each atom (just a number).

• However, for magnetic materials with heavy atoms, you should do the noncollinear magnetic calculations, in which you should open the tag of SOC and set the initial magnetic momentum for each atom (a $1 \times 3$ vector) at the same time.

• You may take a look at the explanation about the initial magnetization in vaspwiki.

• For QE, you could take a look at the following tutorial to do noncollinear magnetic calculations:

• https://www.youtube.com/watch?v=AxfW1iSaz_U

• May it helps.

Answer 2

As a beginner and to the best of my knowledge

1. spin-orbit coupling is irrelevant to the magnetic properties.
2. Spin-orbit coupling is related to relativity which is increasingly important for heavier for elements around Krypton and onward, and also if you're aiming for high precision.
3. For convergence, try to increase electron_maxstep >> 100 (maximum number of iterations in a scf step). and set conv_thr (Convergence threshold for selfconsistency) to the default value 1e-6.