[Pw_forum] ecutvcut in Hybrid functionals

[Jianpeng Liu]

Hi Leonardo,

I really do not know about this. For security, I followed the example and
set ecutvcut to be zero for a HSE hybrid functional.

best,
Jianpeng

> 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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