[QE-users] Stress in hybrid functional calculations

[Audrius Alkauskas]

Dear all,

We have started testing the Quantum Espresso implementation of screened 
Fock exchange (HSE functional), benchmarking against VASP results (the 
input file below).

As a test case, we took simple bulk diamond. If we calculate the energy 
as a function of the lattice constant, the resulting equilibrium lattice 
parameter (3.555 A) and bulk modulus are very similar to the results 
from VASP, which is nice. However, the calculated stress for the 
equilibrium lattice parameter is still large (~30 kBar).  The stress is 
linear as a function of the lattice constant and is ~0 for the lattice 
constant 3.585 A, yielding an offset of about 0.03 A with respect to 
what I assume is the "correct" HSE value. The value of 3.585 A is much 
closer to the PBE result.  Are there still some issues when calculating 
stresses with hybrids or we do something wrong? Energies seem OK, 
stresses not (the error is not huge, but still...)

If anybody has any insights, I would be happy to get receive them.

Best,
Audrius

***
Input
***

&CONTROL
    calculation      = 'scf'
    title            = 'carbon-cubic-hse-3.556'
    verbosity        = 'high'
    nstep            = 100
    tstress          = .true.
    tprnfor          = .true.
    outdir = '/home/vytautas/HSE/C/relax/3.556'
    etot_conv_thr    = 1e-06
    forc_conv_thr    = 1e-05
    pseudo_dir       = '.'
/
&SYSTEM
    ibrav            = 1
    celldm           = 6.719866103476117
    nat              = 8
    ntyp             = 1
    nbnd             = 24
    ecutwfc          = 100
    ecutrho          = 400
    ecutfock         = 400
    input_dft        = 'hse'
    exxdiv_treatment = 'gygi-baldereschi'
    x_gamma_extrapolation = .true.
    nqx1             = 4
    nqx2             = 4
    nqx3             = 4
    celldm(1)        = 6.719866131544066
/
&ELECTRONS
    electron_maxstep = 300
    conv_thr         = 1e-08
/
&IONS
/
&CELL
/

ATOMIC_SPECIES
C 12.011 C.upf

K_POINTS automatic
4 4 4  0 0 0

ATOMIC_POSITIONS angstrom
C 0.0000000000 0.0000000000 0.0000000000
C 1.7780000000 1.7780000000 0.0000000000
C 0.8890000000 0.8890000000 0.8890000000
C 2.6670000000 2.6670000000 0.8890000000
C 0.0000000000 1.7780000000 1.7780000000
C 1.7780000000 0.0000000000 1.7780000000
C 0.8890000000 2.6670000000 2.6670000000
C 2.6670000000 0.8890000000 2.6670000000



-- 
Audrius Alkauskas
Center for Physical Sciences and Technology (FTMC)
Vilnius, Lithuania

email: audrius.alkauskas at ftmc.lt
phone: +370 612 22281
www.puntukas.com / www.ftmc.lt

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