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 <[email protected]> on behalf of Michal Krompiec <[email protected]> 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 <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>> on behalf of Giuseppe Mattioli <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>>: > 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 >> <[email protected]<mailto:[email protected]> >> <mailto:[email protected]>> 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 >> <[email protected]<mailto:[email protected]> >> <mailto:[email protected]>> 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> >> <http://www.max-centre.eu/quantum-espresso>) >> users mailing list >> [email protected]<mailto:[email protected]> >> <mailto:[email protected]> >> 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> >> <http://www.max-centre.eu/quantum-espresso>) >> users mailing list >> [email protected]<mailto:[email protected]> >> <mailto:[email protected]> >> 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<http://www.max-centre.eu/quantum-espresso>) >> users mailing list >> [email protected]<mailto:[email protected]> >> https://lists.quantum-espresso.org/mailman/listinfo/users >> > > -- > Lorenzo Paulatto - Paris > _______________________________________________ > Quantum ESPRESSO is supported by MaX > (www.max-centre.eu/quantum-espresso<http://www.max-centre.eu/quantum-espresso>) > users mailing list > [email protected]<mailto:[email protected]> > 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: <[email protected]<mailto:[email protected]>> _______________________________________________ Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso<http://www.max-centre.eu/quantum-espresso>) users mailing list [email protected]<mailto:[email protected]> 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 [email protected]<mailto:[email protected]> https://lists.quantum-espresso.org/mailman/listinfo/users
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