Dear Quantum ESPRESSO users,
I hope you are well.
I am carrying out DFT calculations on cuprous oxide, Cu2O, using Quantum
ESPRESSO. My aim is to study how the electronic band gap changes with
Hubbard U applied to the Cu 3d states.
I would like to kindly ask for advice on whether my input setup is
reasonable, especially regarding the old-style DFT+U syntax,
pseudopotentials, and use of the Hubbard alpha parameter.
I have attached one representative input file for the vc-relax
calculation at U = 1 eV, since this file includes the Hubbard U setup.
The same workflow is then repeated for U = 2 to 6 eV by changing the
value of Hubbard_U(1). For U = 0, no Hubbard correction is applied.
My calculation setup is as follows:
* Material: Cu2O, cubic cuprite structure
* Exchange-correlation functional: PBE
* Pseudopotentials:
o Cu.pbe-dn-rrkjus_psl.1.0.0.UPF
o O.pbe-n-rrkjus_psl.1.0.0.UPF
* ecutwfc = 60 Ry
* ecutrho = 480 Ry
* k-points for vc-relax and scf: 6 6 6 0 0 0
* k-points for nscf/DOS: 12 12 12 0 0 0
* Number of atoms: 6
* Number of bands in nscf: 40
For U = 1 eV, the relevant part of my &SYSTEM block is:
&SYSTEM
ibrav = 1
a = 4.25220
nat = 6
ntyp = 2
ecutwfc = 60
ecutrho = 480
occupations = 'fixed'
lda_plus_u = .true.
Hubbard_U(1) = 1.0
Hubbard_alpha(1) = -0.08
/
My ATOMIC_SPECIES block is:
ATOMIC_SPECIES
Cu 63.54600 Cu.pbe-dn-rrkjus_psl.1.0.0.UPF
O 15.99940 O.pbe-n-rrkjus_psl.1.0.0.UPF
Therefore, Hubbard_U(1) and Hubbard_alpha(1) are intended to apply to Cu.
The relaxed structure remains cubic, and the calculated direct band gap
at Γ increases from about 0.46 eV at U = 0 to about 0.64 eV at U = 6 eV.
I understand that PBE/PBE+U may still underestimate the experimental
Cu2O band gap, but I would like to confirm whether the input syntax and
general setup are technically correct.
My specific questions are:
1. Is it reasonable to apply Hubbard U only to Cu 3d states in Cu2O
using this old syntax?
2. Is the use of Hubbard_alpha(1) = -0.08 appropriate in a production
DFT+U calculation, or should it only be used for
linear-response/perturbative calculations?
3. Are there any concerns with using occupations = 'fixed' for Cu2O in
vc-relax/scf and occupations = 'tetrahedra' for nscf/DOS?
4. Is nbnd = 40 reasonable for a 6-atom Cu2O cell with 56 electrons for
band-structure and DOS analysis?
5. Are there any syntax issues or methodological concerns that I should
correct before finalizing the calculations?
I would be grateful for any advice or corrections from experienced users.
Kind regards,
Stephen
&CONTROL
calculation = 'vc-relax'
restart_mode = 'from_scratch'
wf_collect = .true.
outdir = './tmp/'
pseudo_dir = './pseudo/'
prefix = 'Cu2O'
verbosity = 'high'
forc_conv_thr = 1.0D-4
etot_conv_thr = 1.0D-4
nstep = 100
/
&SYSTEM
ibrav = 1
a = 4.25220
nat = 6
ntyp = 2
ecutwfc = 60
ecutrho = 480
occupations = 'fixed'
lda_plus_u = .true.
Hubbard_U(1) = 1.0
Hubbard_alpha(1) = -0.08
/
&ELECTRONS
mixing_beta = 0.3
conv_thr = 1.0d-8
/
&IONS
ion_dynamics = 'bfgs'
/
&CELL
cell_dynamics = 'bfgs'
press = 0.0
press_conv_thr = 0.5
cell_dofree = 'all'
/
ATOMIC_SPECIES
Cu 63.54600 Cu.pbe-dn-rrkjus_psl.1.0.0.UPF
O 15.99940 O.pbe-n-rrkjus_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
O 0.000000 0.000000 0.000000
O 0.500000 0.500000 0.500000
Cu 0.250000 0.250000 0.250000
Cu 0.250000 0.750000 0.750000
Cu 0.750000 0.250000 0.750000
Cu 0.750000 0.750000 0.250000
K_POINTS (automatic)
6 6 6 0 0 0
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