[QE-users] Time-reversal broken in DFT+U calculation without starting_magnetization
Dominik Gresch (Experis AG)
a-dogres at microsoft.com
Wed Jan 13 18:28:43 CET 2021
Dear QE community,
I am investigating a system with noncollinear spin and +U correction, but without starting_magnetization. Due to the following sentence in the pw.x input description, I would expect this calculation to exhibit time-reversal symmetry:
In the spin-orbit case starting with zero starting_magnetization on all atoms imposes time reversal symmetry. The magnetization is never calculated and kept zero (the internal variable domag is .FALSE.).
However, the states at k=Gamma are not Kramers pairs (doubly degenerate), indicating that time-reversal symmetry is broken.
To illustrate the behavior on a simplified example, I have modified the Fe calculation from the test suite as follows [1]:
* Remove the starting_magnetization and related inputs
* Create a 2x2x2 supercell and randomly displace the atoms to break all spatial symmetries (using ASE's rattle method with stddev=0.01 Angstrom) [2]
* Reduce the hubbard_U and hubbard_J by a factor of 2, to (presumably?) tune the system out of the ferromagnetic regime [3]
The resulting input files can be found in the attachments, or pasted below. With +U correction [with_u.in], the energies at Gamma [4] are split by ~0.06meV on average, and up to ~0.46 meV. Without +U [without_u.in], the average split is ~3.6e-08 meV, and the maximum is ~3.6e-07 meV.
I would appreciate any input on the following questions:
* Does QE take time-reversal symmetry into account explicitly (by symmetrizing the relevant quantities), or implicitly (by selection of symmetry-inequivalent k-points)?
* If the latter, is there any way to explicitly enforce time-reversal symmetry?
* Is there anything about +U correction in particular that could break time-reversal symmetry (on a technical level, not because it tends to produce magnetic states)?
* Or is there anything about this combination of attributes that means this calculation doesn't make sense?
Thank you and best regards,
Dominik
---
Dr. Dominik Gresch
Simulation Engineer
Microsoft Quantum
[1] This isn't expected to be a physically correct calculation.
[2] This is to get rid of any degeneracies enforced by symmetries other than time-reversal.
[3] Without this change the calculation didn't converge without starting_magnetization. This wasn't a problem for the system I am trying to study (which I unfortunately cannot share), but maybe it is somehow related to the reason why time-reversal is broken?
[4] Taking into account only occupied states, since the unoccupied ones are converged to lower accuracy.
Version details: PWSCF v6.6, compiled with GCC v9.2.0, FFTW v3.3.8, MVAPICH2 v2.3.4, Netlib-SCALAPACK v2.1.0, OPENBLAS v0.3.10, run on Zen2 architecture
[with_u.in]
&CONTROL
calculation = 'scf'
disk_io = 'low'
outdir = './out/'
prefix = 'aiida'
pseudo_dir = './pseudo/'
verbosity = 'low'
/
&SYSTEM
degauss = 1.0000000000d-02
ecutrho = 280
ecutwfc = 35
hubbard_j(1,1) = 8.7500000000d-01
hubbard_j(2,1) = 0.0000000000d+00
hubbard_u(1) = 1.1000000000d+00
ibrav = 0
lda_plus_u = .true.
lda_plus_u_kind = 1
lspinorb = .true.
nat = 8
nbnd = 142
noncolin = .true.
ntyp = 1
occupations = 'smearing'
smearing = 'mv'
/
&ELECTRONS
conv_thr = 1.0000000000d-08
/
ATOMIC_SPECIES
Fe 55.845 Fe.rel-pbe-spn-rrkjus_psl.0.2.1.UPF
ATOMIC_POSITIONS angstrom
Fe 0.0049671415 -0.0013826430 0.0064768854
Fe 1.4502302986 1.4326584663 -1.4373413696
Fe 1.4507921282 -1.4273256527 1.4303052561
Fe 2.8754256004 -0.0046341769 -0.0046572975
Fe -1.4325803773 1.4158671976 1.4177508217
Fe -0.0056228753 2.8598716888 0.0031424733
Fe -0.0090802408 -0.0141230370 2.8846564877
Fe 1.4327422370 1.4356752820 1.4207525181
K_POINTS automatic
4 4 4 0 0 0
CELL_PARAMETERS angstrom
-2.8700000000 2.8700000000 2.8700000000
2.8700000000 -2.8700000000 2.8700000000
2.8700000000 2.8700000000 -2.8700000000
[without_u.in]
&CONTROL
calculation = 'scf'
disk_io = 'low'
outdir = './out/'
prefix = 'aiida'
pseudo_dir = './pseudo/'
verbosity = 'low'
/
&SYSTEM
degauss = 1.0000000000d-02
ecutrho = 280
ecutwfc = 35
ibrav = 0
lspinorb = .true.
nat = 8
nbnd = 142
noncolin = .true.
ntyp = 1
occupations = 'smearing'
smearing = 'mv'
/
&ELECTRONS
conv_thr = 1.0000000000d-08
/
ATOMIC_SPECIES
Fe 55.845 Fe.rel-pbe-spn-rrkjus_psl.0.2.1.UPF
ATOMIC_POSITIONS angstrom
Fe 0.0049671415 -0.0013826430 0.0064768854
Fe 1.4502302986 1.4326584663 -1.4373413696
Fe 1.4507921282 -1.4273256527 1.4303052561
Fe 2.8754256004 -0.0046341769 -0.0046572975
Fe -1.4325803773 1.4158671976 1.4177508217
Fe -0.0056228753 2.8598716888 0.0031424733
Fe -0.0090802408 -0.0141230370 2.8846564877
Fe 1.4327422370 1.4356752820 1.4207525181
K_POINTS automatic
4 4 4 0 0 0
CELL_PARAMETERS angstrom
-2.8700000000 2.8700000000 2.8700000000
2.8700000000 -2.8700000000 2.8700000000
2.8700000000 2.8700000000 -2.8700000000
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