Dear Iurii, thank you very much for the explanation and for the references.
Best, Michal On Wed, 8 Apr 2020 at 21:09, Timrov Iurii <iurii.tim...@epfl.ch> wrote: > Dear Michal, > > > > 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? > > > I am not familiar with GWW, but as far as I know it can be used only with > a gamma point (k=0). I do not know what are other limitations, and whether > you can apply it to compute optimal absorption spectra in solids. You may > contact the developer Paolo Umari. > > > > Is it why TDDFT with hybrids for periodic systems is missing in QE - > because GW is better and tractable so why bother with TDDFT? > > > It is a long story, but let me write briefly what I know (please correct > me if I am wrong). I suggest to check the literature on this topic. > > > GW+BSE is the standard approach to compute optical absorption spectra in > periodic systems (semiconductors). This is so because the non-local > interactions are present, and because the dielectric function (r-dependent > and w-dependent) is present in the formalism which describes the > dielectric properties of the periodic system. > > > TDDFT in the adiabatic approximation with local/semilocal functionals > (like LDA/GGA) is not accurate for a description of optical absorption > spectra of periodic solids (please check the literature). However, TDDFT > with non-local functionals is an interesting alternative to GW+BSE. > TDDFT@PBE0 improves considerably w.r.t TDDFT@PBE, but still with just a > constant (alpha=0.25) in the EXX part it is hard to do a descent job for > solids (actually alpha is related to 1/epsilon : Phys. Rev. B 89, 195112 > (2014)). Much better is to use TDDFT in combination with rage-separated > hybrid functionals, where 1/epsilon is not just a constant but a function > 1/epsilon(r) (with models for epsilon(r)), which better represents the > dielectric properties of the solid (Phys. Rev. B 93, 235106 (2016), check > in particular the introduction in this reference about the history of > various non-local XC kernels). > > > 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-boun...@lists.quantum-espresso.org> on behalf of > Michal Krompiec <michal.kromp...@gmail.com> > *Sent:* Wednesday, April 8, 2020 9:40:01 PM > > *To:* Quantum ESPRESSO users Forum > *Subject:* Re: [QE-users] epsilon.x and hybrids > > 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.tim...@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-boun...@lists.quantum-espresso.org> on behalf of >> Giuseppe Mattioli <giuseppe.matti...@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 <paul...@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 <mhe...@sfu.ca >> >> <mailto:mhe...@sfu.ca <mhe...@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.kromp...@gmail.com <mailto:michal.kromp...@gmail.com >> <michal.kromp...@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 >> >> <http://www.max-centre.eu/quantum-espresso>) >> >> users mailing list users@lists.quantum-espresso.org >> >> <mailto:users@lists.quantum-espresso.org >> <users@lists.quantum-espresso.org>> >> >> https://lists.quantum-espresso.org/mailman/listinfo/users >> >> >> >> _______________________________________________ >> >> Quantum ESPRESSO is supported by MaX >> >> (www.max-centre.eu/quantum-espresso >> >> <http://www.max-centre.eu/quantum-espresso>) >> >> users mailing list users@lists.quantum-espresso.org >> >> <mailto:users@lists.quantum-espresso.org >> <users@lists.quantum-espresso.org>> >> >> https://lists.quantum-espresso.org/mailman/listinfo/users >> >> >> >> >> >> >> >> -- >> >> 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 ( >> www.max-centre.eu/quantum-espresso) >> >> users mailing list users@lists.quantum-espresso.org >> >> https://lists.quantum-espresso.org/mailman/listinfo/users >> >> >> > >> > -- >> > Lorenzo Paulatto - Paris >> > _______________________________________________ >> > Quantum ESPRESSO is supported by MaX ( >> www.max-centre.eu/quantum-espresso) >> > users mailing list users@lists.quantum-espresso.org >> > https://lists.quantum-espresso.org/mailman/listinfo/users >> >> >> >> 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.matti...@ism.cnr.it> >> >> _______________________________________________ >> Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) >> users mailing list users@lists.quantum-espresso.org >> https://lists.quantum-espresso.org/mailman/listinfo/users >> _______________________________________________ >> Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) >> users mailing list users@lists.quantum-espresso.org >> https://lists.quantum-espresso.org/mailman/listinfo/users > > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) > users mailing list users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users
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