[Pw_forum] Fwd: Error in Hybrid Exc calculations

[Debnath Talukdar]

---------- Forwarded message ----------
From: Debnath Talukdar <debnath.talukdar at students.iiserpune.ac.in>
Date: Tue, Sep 13, 2016 at 4:25 PM
Subject: Error in Hybrid Exc calculations
To: PWSCF Forum <pw_forum at pwscf.org>


Hi,
   I was trying to do hybrid calculations in quantum espresso. The
calculation stopped by printing the following message

    Error in routine electrons (1):
     dexx is negative!   Check that exxdiv_treatment is appropriate for the
system

As I know that in hybrid Exc calculation to solve the poisson equation in
reciprocal space their is
fourier transform of density from real to q space. That colomb potential in
poisson equation is diverge at q=0. To remove this divergencies in coulomb
potential in q space is adds a small G to
denomenetor(q+G); known as   gygi-baldereschi method, but it is
appropriate  for cubic and quasi cubic supercell (Gygi-Baldereschi PRB 34,
4405 (1986)).

At first I have used  gygi-baldereschi method for my system that is
hexagonal metallic but I have found above error then I have changed this
method from 'gygi-baldereschi' to 'vcut_ws' that is appropriate for
anisotropic supercell but I still got that same error message.

After that I have changed Hybrid functional from HSE to gau-PBE which used
exxdiv_treatment='none' and x_gamma_extrapolation=.false. but I still got
that same error
 message again.


Can anybody give any suggestion that how can I solve this problem ?

I have also doubt about following...

1)As we all know that hybrid calculation is very expansive so how can I
reduce different   parameter in Hybrid calculation as compared to normal
scf calculations.

   Such that in my normal scf with DFT Exc I have used k -mesh 21x21x1, scf
threshold 10-8Ry,
   then how can I reduce these parameters in Hybrid calculations; is it
arbitrary?

2)How can I reduce ecutfock to speed up the calculation. Currently I have
used ecutfock=ecutrho (for my system ecutrho=10*ecutwfc), is their is any
way to reduce this kinetic energy cutoff for fock (exchange) operator at
the cost of loss in accuracy in my calculations.

3) In our pwscf input file we give explicitly effect of
exchange-correlation functional through card
input_dft, I am doubt that how can I gave both vdW(rev-vdw-b86r) functional
with hybrid calculations(where input_dft='HSE').

I have read papers but I am unable to solve these issues can anyone will
give any other papers that how can I solve this.

Below I have attached input, output files of my calculations.
1)PRB 34, 4405 (1986)
2)PHYSICAL REVIEW B 80, 115205 ͑2009͒
3)PHYSICAL REVIEW B 74, 121102͑R͒ ͑2006͒
4)JCP 118, 8207 (2003)
5)THE JOURNAL OF CHEMICAL PHYSICS 134, 024116 (2011)




Thanks
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