[QE-users] epsilon.x and hybrids

Wed Apr 8 21:40:01 CEST 2020

BTW would you say that for calculation of the dielectric function of
not-so-large (whatever this means...) periodic systems it is better to try
GW with the GWW code instead? Is it why TDDFT with hybrids for periodic
systems is missing in QE - because GW is better and tractable so why bother
with TDDFT?
Best,
Michal

On Wed, Apr 8, 2020 at 19:15 Timrov Iurii <iurii.timrov at epfl.ch> wrote:

> Dear All,
>
>
> > Do I remember correctly that epsilon.x also does not take into account
> the nonlocal pseudopotential contribution?
>
>
> Correct
>
>
> > ...there used to be an option in the  turbo_davidson.x and
> turbo_lanczos.x codes, namely no_hxc=.true.,  which permits an
> independent-electron calculation.
>
>
> Correct. By default, the matrix element of the dipole operator is computed
> (in reciprocal space) via the matrix element of the commutator that Paolo
> mentioned [H,x] (see Eq.(14) in S. Baroni and R. Resta, Phys. Rev. B 33,
> 7017 (1986)): but in this case there is a kinetic term and the part coming
> from the non-local part of a pseudopotential. In epsilon.x, only the
> kinetic term is present, while in TDDFT codes both terms are present (the
> second term was implement long ago in the Phonon code to compute the
> dielectric tensor). But still, in the case of hybrid functionals, in [H,x]
> there is another term which is missing - the commutator with another
> non-local potential which is EXX. Stefano Baroni and I developed a way how
> to compute this missing term several years ago: I implemented it in QE and
> it worked well, but I never had time to release it and publish some paper
> about it. But there is a workaround for finite systems: the matrix element
> of the dipole operator can be computed in real space based on the
> observation that the charge density of the finite system decays fast
> outside the finite system, and hence the non-periodicity problem of the
> dipole operator is no longer a problem. But this trick in real space is not
> gonna work for periodic systems, because the charge density if non-zero in
> the whole simulation cell. Thus, the only way to overcome this problem is
> to implement the missing term for hybrids in [H,x].
>
>
> Greetings,
>
> Iurii
>
>
> --
> Dr. Iurii Timrov
> Postdoctoral Researcher
> STI - IMX - THEOS and NCCR - MARVEL
> Swiss Federal Institute of Technology Lausanne (EPFL)
> CH-1015 Lausanne, Switzerland
> +41 21 69 34 881
> http://people.epfl.ch/265334
> ------------------------------
> *From:* users <users-bounces at lists.quantum-espresso.org> on behalf of
> Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it>
> *Sent:* Wednesday, April 8, 2020 7:42:35 PM
> *To:* Quantum ESPRESSO users Forum
> *Subject:* Re: [QE-users] epsilon.x and hybrids
>
>
> Dear all
> I don't want to raise the confusion level, so please correct me if I'm
> wrong... If you want to calculate a heavily approximate absorption
> spectrum of a (large and non-symmetrical) periodic system after a
> ground state hybrid calculation, there used to be an option in the
> turbo_davidson.x and turbo_lanczos.x codes, namely no_hxc=.true.,
> which permits an independent-electron calculation. At least, hybrid
> functionals and Gamma ground states should be properly treated by such
> codes, resulting in an absorption spectrum compatible with those
> obtained by using epsilon.x, which, AFAIK, calculates
> <occupied|r|virtual> contributions to the absorption spectrum.
> However, I don't know how much this kind of calculation is expensive
> for large supercells. Of course if you are not interested in
> absorption, then my suggestion is nonsense...
> HTH
> Giuseppe
>
> Quoting Lorenzo Paulatto <paulatz at gmail.com>:
>
> > Also, epsilon.x cannot use symmetry-reduced grids, which would be a
> > huge wast of time with hybrids, but you can use open_grid.x after
> > the pw.x calculation too obtain the full grid and work around this
> > problem.
> >
> > cheers
> >
> > On 4/8/20 6:38 PM, Paolo Giannozzi wrote:
> >> I think epsilon.x assumes that the dipole element of x can be
> >> computed using [H,x]=p\hbar/m. The exchange potential is nonlocal,
> >> so its commutator with x will yield an additional term that is not
> >> accounted for. Not sure how important it is in practice. Do I
> >> remember correctly that epsilon.x also does not take into account
> >> the nonlocal pseudopotential contribution?
> >>
> >> Paolo
> >>
> >> On Wed, Apr 8, 2020 at 4:29 PM Manu Hegde <mhegde at sfu.ca
> >> <mailto:mhegde at sfu.ca <mhegde at sfu.ca>>> wrote:
> >>
> >>    Hi Michal,
> >>    Yes, it is possible.I did use both supercell and hybrid
> >>    calculations. It did work.
> >>    Manu
> >>
> >>    On Wed, Apr 8, 2020 at 10:08 AM Michal Krompiec
> >>    <michal.krompiec at gmail.com <mailto:michal.krompiec at gmail.com
> <michal.krompiec at gmail.com>>> wrote:
> >>
> >>        Hello,
> >>        Is it possible to use epsilon.x on results of a calculation with
> a
> >>        hybrid functional (supercell, gamma point only)?
> >>
> >>        Thanks,
> >>
> >>        Michal Krompiec
> >>        Merck KGaA
> >>        _______________________________________________
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> >>
> >>
> >> --
> >> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> >> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> <https://www.google.com/maps/search/Udine,+via+delle+Scienze+208,+33100+Udine,+Italy?entry=gmail&source=g>
> >> Phone +39-0432-558216, fax +39-0432-558222
> >>
> >>
> >> _______________________________________________
> >> Quantum ESPRESSO is supported by MaX (
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> >
> > --
> > Lorenzo Paulatto - Paris
> > _______________________________________________
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>
>
> GIUSEPPE MATTIOLI
> CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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>
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