Spin-polarization and SOC
Non-spin-polarized Calculations
Setting of “nspin 1” in INPUT file means calculation with non-polarized spin. In this case, electrons with spin up and spin down have same occupations at every energy states, weights of bands per k point would be double.
Collinear Spin Polarized Calculations
Setting of “nspin 2” in INPUT file means calculation with polarized spin along z-axis. In this case, electrons with spin up and spin down will be calculated respectively, number of k points would be doubled. Potential of electron and charge density will separate to spin-up case and spin-down case.
Magnetic moment Settings in STRU files are not ignored until “nspin 2” is set in INPUT file
When “nspin 2” is set, the screen output file will contain magnetic moment information. e.g.
ITER TMAG AMAG ETOT(eV) EDIFF(eV) DRHO TIME(s)
GE1 4.16e+00 4.36e+00 -6.440173e+03 0.000000e+00 6.516e-02 1.973e+01
where “TMAG” refers to total magnetization and “AMAG” refers to average magnetization. For more detailed orbital magnetic moment information, please use Mulliken charge analysis.
Constraint DFT for collinear spin polarized calculations
For some special need, there are two method to constrain electronic spin.
“ocp” and “ocp_set” If “ocp=1” and “ocp_set” is set in INPUT file, the occupations of states would be fixed by “ocp_set”, this method is often used for excited states calculation. Be careful that: when “nspin=1”, spin-up and spin-down electrons will both be set, and when “nspin=2”, you should set spin-up and spin-down respectively.
“nupdown” If “nupdown” is set to non-zero, number of spin-up and spin-down electrons will be fixed, and Fermi energy level will split to E_Fermi_up and E_Fermi_down. By the way, total magnetization will also be fixed, and will be the value of “nupdown”.
DeltaSpin The
DeltaSpin
function as proposed by Zefeng Cai and Ben Xu, et al. arXiv:2208.04551v6 has been implemented in ABACUS in the LCAO basis for thenspin 2
case. In order to use this function, the following parameters are needed to be set in the input file, for example:
#deltaspin
sc_mag_switch 1
decay_grad_switch 0
sc_thr 1e-7
nsc 150
nsc_min 2
sc_file sc.json
alpha_trial 0.01
sccut 3
The explanation of each input paramters has been explained in the Noncollinear Spin Polarized Calculations section.
An example of the sc_file json file is shown below:
[
{
"element": "Fe",
"itype": 0,
"ScDecayGrad": 0.9,
"ScAtomData": [
{
"index": 0,
"lambda": 0.0,
"target_mag": 2.0,
"constrain": 1
},
{
"index": 1,
"lambda": 0,
"target_mag": 2.0,
"constrain": 1
}
]
}
]
Please refer the Noncollinear Spin Polarized Calculations section for the explanation of each input paramters. The difference is that lambda
, target_mag
, and constrain
are scalars instead of vectors. Simple examples are provided in the abacus-develop/examples/spin_polarized
directory.
Noncollinear Spin Polarized Calculations
The spin non-collinear polarization calculation corresponds to setting “noncolin 1”, in which case the coupling between spin up and spin down will be taken into account. In this case, nspin is automatically set to 4, which is usually not required to be specified manually. The weight of each band will not change, but the number of occupied states will be double. If the nbands parameter is set manually, it is generally set to twice what it would be when nspin<4.
In general, non-collinear magnetic moment settings are often used in calculations considering SOC effects. When “lspinorb 1” in INPUT file, “nspin” is also automatically set to 4. Note: different settings for “noncolin” and “lspinorb” correspond to different calculations:
noncolin=0 lspinorb=0 nspin<4 : Non-collinear magnetic moments and SOC effects are not considered.
noncolin=0 lspinorb=0 nspin=4 : Actualy same as the above setting, but the calculation will be larger. So the setting is not recommended.
noncolin=1 lspinorb=0 : Non-collinear magnetic moments are considered but SOC effects are not considered
noncolin=0 lspinorb=1 : The SOC effect is considered but the magnetic moment is limited to the Z direction
noncolin=1 lspinorb=1 : The SOC effect and non-collinear magnetic moment are both calculated.
Constraint Spin functionality for noncollinear spin polarized calculations
The DeltaSpin
function as proposed by Zefeng Cai and Ben Xu, et al. arXiv:2208.04551v6 has been implemented in ABACUS in the LCAO basis. In order to use this function, the following parameters are needed to be set in the input file, for example:
#deltaspin
sc_mag_switch 1
decay_grad_switch 1
sc_thr 1e-7
nsc 150
nsc_min 2
sc_file sc.json
alpha_trial 0.01
sccut 3
sc_mag_switch
is the switch of deltaspin functionality; decay_grad_switch
is the switch of decay gradient method; sc_thr
is the threshold of the spin constraint atomic magnetic moment in unit of Bohr Mag (\muB); nsc
is the number of self-consistent iterations; nsc_min
is the minimum number of self-consistent iterations; sc_file
is the file name of the spin constraint parameters; alpha_trial
is the initial trial step size for lambda in eV/uB^2; sccut
restriction of step size in eV/uB.
An example of the sc_file json file is shown below:
[
{
"element": "Fe",
"itype": 0,
"ScDecayGrad": 0.9,
"ScAtomData": [
{
"index": 0,
"lambda": [0, 0, 0],
"target_mag": [2.0, 0.0, 0.0],
"constrain": [1,1,1]
},
{
"index": 1,
"lambda": [0, 0, 0],
"target_mag_val": 2.0,
"target_mag_angle1": 80.0,
"target_mag_angle2": 0.0,
"constrain": [1,1,1]
}
]
}
]
The sc_file json file is a list of elemental data in total. For each element, the user should specify its name, the itype
parameter should be in accord with STRU
file and start from 0. ScDecayGrad
is a parameter for each element in unit of (uB^2/eV), this parameter needs to be determined for different element, for example, 0.9 uB^2/eV is an appropriate value for BCC-Fe according to Zefeng Cai’s tests. ScAtomData
specifies spin constraining parameters for each atom, the index
starts from 0 and corresponds atomic order in the STRU
file. lambda
is a 3d vector for each atom, and it is recommended to set to [0.0, 0.0, 0.0] for all atoms. Users have two optional choices to set the target magnetic moments for each atom, i.e., by a 3d vector or by angles. If the target_mag
is set, the target_mag_val
and target_mag_angle1
and target_mag_angle2
will be ignored. The target_mag
is a 3d vector in unit of Bohr Mag (\muB), and the target_mag_val
is a scalar value in unit of Bohr Mag (\muB), target_mag_angle1
and target_mag_angle2
are two angles in unit of degree. The constrain
is a 3d vector, if the corresponding element is set to 1, the corresponding component of the magnetic moment will be constrained, otherwise, it will be free. Note that the initial atomic magnetic moments are still set in the STRU
file. Simple examples are provided in the abacus-develop/examples/noncollinear
directory. One should set noncolliear
to 1 to run the DeltaSpin function, lspinorb=1
is not mandatory, but it is recommended to set to 1 to get more accurate results.
For the continuation job
Continuation job for “nspin 1” need file “SPIN1_CHG.cube” which is generated by setting “out_chg=1” in task before. By setting “init_chg file” in new job’s INPUT file, charge density will start from file but not atomic.
Continuation job for “nspin 2” need files “SPIN1_CHG.cube” and “SPIN2_CHG.cube” which are generated by “out_chg 1” with “nspin 2”, and refer to spin-up and spin-down charge densities respectively. It should be note that reading “SPIN1_CHG.cube” only for the continuation target magnetic moment job is not supported now.
Continuation job for “nspin 4” need files “SPIN%s_CHG.cube”, where %s in {1,2,3,4}, which are generated by “out_chg 1” with any variable setting leading to ‘nspin’=4, and refer to charge densities in Pauli spin matrixes. It should be note that reading charge density files printing by ‘nspin’=2 case is supported, which means only \(\sigma_{tot}\) and \(\sigma_{z}\) are read.
SOC Effects
SOC
lspinorb
is used for control whether or not SOC(spin-orbit coupling) effects should be considered.
Both basis_type=pw
and basis_type=lcao
support scf
and nscf
calculation with SOC effects.
Atomic forces and cell stresses can not be calculated with SOC effects yet.
Pseudopotentials and Numerical Atomic Orbitals
For Norm-Conserving pseudopotentials, there are differences between SOC version and non-SOC version.
Please check your pseudopotential files before calculating. In PP_HEADER
part, keyword has_so=1
and relativistic="full"
refer to SOC effects have been considered, which would lead to different PP_NONLOCAL
and PP_PSWFC
parts. Please be careful that relativistic="full"
version can be used for SOC or non-SOC calculation, but relativistic="scalar"
version only can be used for non-SOC calculation. When full-relativistic pseudopotential is used for non-SOC calculation, ABACUS will automatically transform it to scalar-relativistic version.
Numerical atomic orbitals in ABACUS are unrelated with spin, and same orbital file can be used for SOC and non-SOC calculation.
Partial-relativistic SOC Effect
Sometimes, for some real materials, both scalar-relativistic and full-relativistic can not describe the exact spin-orbit coupling. Artificial modulation can help for these cases.
soc_lambda
, which has value range [0.0, 1.0] , is used for modulate SOC effect. In particular, soc_lambda 0.0
refers to scalar-relativistic case and soc_lambda 1.0
refers to full-relativistic case.