[QE-users] epsilon.x and hybrids

Manu Hegde mhegde at sfu.ca
Wed Apr 8 20:30:13 CEST 2020 

Hello All,
Thank you very much.
Giuseppe: Thanks for clarifying. You have suggested me to use turbo.x for
absorption spectrum over epsilon.x for periodic systems, while ago. I do
remember now.
Thanks
Manu
(McMaster University)


On Wed, Apr 8, 2020 at 2:21 PM Michal Krompiec <michal.krompiec at gmail.com>
wrote:

> Dear All,
> Thank you very much, this explains that it can't be done for my system
> and closes the case for now :(. It would be great if someone
> implemented the missing term for hybrids in [H,x].
> Best,
> Michal
>
> On Wed, 8 Apr 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 comput
>  ed in re
>  al 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>> 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>>
> 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
> > >>        _______________________________________________
> > >>        Quantum ESPRESSO is supported by MaX
> > >>        (www.max-centre.eu/quantum-espresso
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> > >>
> > >>    _______________________________________________
> > >>    Quantum ESPRESSO is supported by MaX
> > >>    (www.max-centre.eu/quantum-espresso
> > >>    <http://www.max-centre.eu/quantum-espresso>)
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> > >>
> > >>
> > >>
> > >> --
> > >> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> > >> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> > >> Phone +39-0432-558216, fax +39-0432-558222
> > >>
> > >>
> > >> _______________________________________________
> > >> Quantum ESPRESSO is supported by MaX (
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> > >>
> > >
> > > --
> > > Lorenzo Paulatto - Paris
> > > _______________________________________________
> > > Quantum ESPRESSO is supported by MaX (
> www.max-centre.eu/quantum-espresso)
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> >
> >
> >
> > GIUSEPPE MATTIOLI
> > CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
> > Via Salaria Km 29,300 - C.P. 10
> > I-00015 - Monterotondo Scalo (RM)
> > Mob (*preferred*) +39 373 7305625
> > Tel + 39 06 90672342 - Fax +39 06 90672316
> > E-mail: <giuseppe.mattioli at ism.cnr.it>
> >
> > _______________________________________________
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