[Pw_forum] Hybrid DFT calculations in QE v 6.2 and PAW datasets

[Luc LeBlanc]

Hello,

I have recently been attempting to perform calculations within Quantum ESPRESSO v 6.2 using hybrid PBE0 DFT on a couple of test systems. However, I am encountering the following problem. For the following test case, a diamond crystal structure (input can be found below), I have had the following error show up in the output:

"dexx is negative!  Check that exxdiv_treatment is appropriate for the system, or ecutfock may be too low"

Now, I realize that this error has been reported many times in previous years and is due to the divergence of the exact exchange operator at small values of q points. As I understand it, the current treatment for this diverging term (gygi-baldereschi) is applicable to cells that are ~cubic; but, the diamond crystal is cubic. Suggested workarounds to resolve the error above have been to change this scheme (e.g., truncate the Coulomb potential or use vcut_ws, increase ecutfock, shift the k-point mesh away from the gamma point, etc.). I have exhausted all of these possibilites, none of which have worked for the diamond crystal or many other crystal systems that I have tried. For instance, the same error occurs for the urea crystal structure, but not for a single urea molecule in a large vaccuum (both inputs can be found below) when performing SCF calculations.

Because of this, I have become suspicious of the pseudopotentials that I am using to perform these calculations, i.e., the PBE PAW datasets (e.g., the ones found on the QE website). If I instead use a PBE norm-conserving pseudopotential, the SCF does converge.

Are there any considerations to take into account when generating PAW datasets as opposed to norm-conserving pseudopotentials that would cause this issue? Or is there something in the QE implementation itself which would cause this divergence in exact exchange when PAW datasets are used?

If anyone has insight on the matter, help would be appreciated.

Thank you,
Luc LeBlanc
luc_DOT_leblanc_AT_ dal_DOT_ca
Department of Chemistry
Dalhousie University, Canada

The input file for the diamond crystal structure:

&control
title='diamond',
prefix='diamond',
pseudo_dir='.',
/
&system
ibrav=0,
celldm(1)=1.0,
nat=     8,
ntyp=  1,
ecutwfc=80.0,
ecutrho=800.0,
input_dft='PBE0',
exx_fraction=0.25,
/
&electrons
conv_thr = 1d-8,
/
&ions
/
&cell
/
ATOMIC_SPECIES
c     12.010700 c.UPF

ATOMIC_POSITIONS crystal
c     0.12500000     0.12500000     0.12500000
c     0.12500000     0.62500000     0.62500000
c     0.62500000     0.12500000     0.62500000
c     0.62500000     0.62500000     0.12500000
c     0.87500000     0.87500000     0.87500000
c     0.87500000     0.37500000     0.37500000
c     0.37500000     0.87500000     0.37500000
c     0.37500000     0.37500000     0.87500000

K_POINTS automatic
4 4 4 1 1 1

CELL_PARAMETERS bohr
6.740086198163 0.000000000000 0.000000000000
0.000000000000 6.740086198163 0.000000000000
0.000000000000 0.000000000000 6.740086198163

The inputs for the urea crystal and single urea molecule in a large vaccuum:

&control
title='urea',
prefix='urea',
pseudo_dir='.',
/
&system
ibrav=0,
celldm(1)=1.0,
nat=    16,
ntyp=  4,
ecutwfc=80,
ecutrho=800,
input_dft='PBE0',
exx_fraction=0.25,
/
&electrons
conv_thr = 1d-8,
/
&ions
/
&cell
/
ATOMIC_SPECIES
h      1.007940 h.UPF
c     12.010700 c.UPF
n     14.006700 n.UPF
o     15.999400 o.UPF

ATOMIC_POSITIONS crystal
c     1.00000000     0.50000000     0.32747528
c     0.50000000     0.00000000     0.67252472
h     0.26121365     0.76121365     0.28250193
h     0.76121365     0.73878635     0.71749807
h     0.23878635     0.26121365     0.71749807
h     0.73878635     0.23878635     0.28250193
h     0.14516494     0.64516494     0.95967949
h     0.64516494     0.85483506     0.04032051
h     0.35483506     0.14516494     0.04032051
h     0.85483506     0.35483506     0.95967949
o     1.00000000     0.50000000     0.60019309
o     0.50000000     0.00000000     0.39980691
n     0.14704706     0.64704706     0.17768392
n     0.64704706     0.85295294     0.82231608
n     0.35295294     0.14704706     0.82231608
n     0.85295294     0.35295294     0.17768392

K_POINTS automatic
4 4 4 1 1 1

CELL_PARAMETERS bohr
 10.473277845000     0.000000000000     0.000000000000
  0.000000000000    10.473277845000     0.000000000000
  0.000000000000     0.000000000000     8.826950084000

####

&control
title='urea_mol',
prefix='urea_mol',
pseudo_dir='.',
/
&system
ibrav=0,
celldm(1)=1.0,
nat=     8,
ntyp=  4,
ecutwfc=80,
ecutrho=800,
input_dft='PBE0',
exx_fraction=0.25,
/
&electrons
conv_thr = 1d-8,
/
&ions
/
&cell
/
ATOMIC_SPECIES
h      1.007940 h.UPF
c     12.010700 c.UPF
n     14.006700 n.UPF
o     15.999400 o.UPF

ATOMIC_POSITIONS crystal
c     0.00462173     0.01313468     0.00001026
o     0.03762823     0.07042739     0.99972194
n     0.93159137     0.00837905     0.98441030
h     0.90253872     0.96768568     0.00623664
h     0.90634733     0.05670150     0.98288178
n     0.03721573     0.94767996     0.01598762
h     0.01654374     0.90200798     0.99445500
h     0.09169167     0.95012689     0.01719219

K_POINTS automatic
1 1 1 1 1 1

CELL_PARAMETERS bohr
 35.000000000000     0.000000000000     0.000000000000
  0.000000000000    35.000000000000     0.000000000000
  0.000000000000     0.000000000000    35.000000000000