[Pw_forum] ecutvcut in Hybrid functionals

[Leonardo Matheus]

HSE is "a PBE0 with a cutoff". I don't know if it has been fixed, but some
time ago ecutvcut interfered with that cutoff, making HSE behave just as
PBE0 (i.e., when ecutvcut was on, calculations done with HSE and PBE0 gave
the same results).
Maybe that's why in the examples with HSE was used ecutvcut=0.0. I just
don't know if this is still happening, or if the bug was fixed and just the
example that wasn't updated.

Leonardo Jorge
PhD Student at USP - Brazil
Nanomol Group
http://nanomol.if.usp.br

On Tue, Nov 20, 2012 at 10:48 AM, Jianpeng Liu <jpliu at physics.rutgers.edu>wrote:

> Hi Layla,
>
> Your comments are really really helpful! Thanks a lot!
>
> best,
> Jianpeng
>
> > Dear Jianpeng, ecutvcut is a parameter which is not strictly bonded to
> > hybrid functional parameters, but to strongly anisotropic supercells like
> > in the case of quasi 1D systems and fock like operators. I will be more
> > precise. If you have an hybrid functional AND you want to have the
> > electronic properties of a polymer chain or a nanotube, i.e. if you
> > multiply the cell parameter by the number of kpoint in each direction
> then
> > you obtain an effective supercell which is anisotropic (like 6*6*50),
> then
> > to avoid divergencies for small q vectors in the integration of the fock
> > operator, one has to turn on exxdiv_treatment="vcut_ws" and ecutvcut=1.2
> > or
> > something appropriate (you need to converge with this value). vcut_ws
> > applies a real space cutoff in the wigner Seitz cell of your supercell
> > until a reciprocal space cutoff of ecutvcut. The more is your cell
> > anisotropic bigger has to be ecutvcut. If you are dealing with bulk
> > materials, you do not have to care about this!!!
> >
> > hope it helps cheers
> >
> > Layla
> >
> > 2012/11/19 Jianpeng Liu <jpliu at physics.rutgers.edu>
> >
> >> Dear QE users and developers,
> >>
> >> I am doing hybrid-functional calculations, and I am confused about the
> >> setting of ecutvcut. In the examples of Si, for pbe0-type hybrid
> >> functional, the ecutvcut is set to be 0.7Ry, while for hse-type hybrid
> >> functional with a screened Coulomb potential, ecutvcut is just set as
> >> default which is 0.0Ry. Could anyone explain why? For a typical
> >> semiconductor with s or p valence orbitals, what kind of ecutvcut value
> >> is
> >> recommended?
> >>
> >> Thanks a lot.
> >>
> >> best,
> >> Jianpeng Liu
> >>
> >> -------------------------------------
> >> Ph.D. candidate,
> >> Department of physics and astronomy,
> >> Rutgers University
> >> -------------------------------------
> >>
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