Hi Galvin, Checkout the units of screening parameter. I think HSE06 screening parameter is 0.2 Angstroms^-1. And I think PWSCF accepts screening parameter in Bohr^-1. So 0.0916 would correspond to ~0.173A^-1.
On Fri, May 22, 2015 at 11:41 AM, Khara, Galvin <[email protected]> wrote: > Hey, > > > Thanks for your reply. Using an ecut of 60 Ry, a 8x8x8 kpoint grid (and > also an 8x8x8 nqx grid), and a screening parameter of 0.0916, I got a value > of 1.19 eV using the method you outlined. Is that a bit too good to be > true? Heyd's original thesis quotes a value of 1.31 eV for silicon, but > that was using a screening parameter of 0.15, which may account for the > difference. Have you attempted to obtain bandstructure graph by using a > defined uniform k-point list for the 2nd scf calculation? I'd like to know > how to do this just with the eigenvalues (sorry for the novice questions), > and thanks again for your reply. > > > > Galvin, > University College London > ------------------------------ > *From:* DHIRENDRA VAIDYA <[email protected]> > *Sent:* 20 May 2015 20:40 > > *To:* PWSCF Forum > *Subject:* Re: [Pw_forum] Silicon HSE Problem > > Hi Galvin, > > I am also looking at HSE bandstructure calculaitons, mainly for Group IV > elements from semiconductore point of view. I get from PWSCF forum that its > difficult to obtain the bandstructure using HSE, but bandgap might be easy > to obtain. Try this, > In scf calculations add, nbnd (=8 if using 2 atom FCC lattice for > Silicon). Remove 'occumpations='smearing''. And then do scf calculations > first without HSE and then with HSE with 'restart_mode='restart''. In > output file you will find something like this, > highest occupied, lowest unoccupied level (ev): #some_value1 > #some_value2 > > the difference is the bandgap = #some_value2-#some_value1 > > On Wed, May 20, 2015 at 6:54 PM, Khara, Galvin <[email protected]> > wrote: > >> Hey, >> >> >> I'm getting a value of 5.445 Angstroms for my lattice paramter, which >> is pretty close to the value quoted by Heyd (5.45), and that using a >> different screening parameter (which he says does alter the physical >> properties anyway). The next thing I want to do is calculate the band >> gap, searching through the archives actually gives loads of topics on this >> very problem. However, I'm really new to DFT, is there any chance you could >> provide a method to do this, so far I've understood; >> >> >> (i) - Do a regular scf calculation using an automatic fine k-point mesh >> (I'm running an 8x8x8 simulation now) >> >> >> (ii) - Run another scf calculation with a defined k-point list (does >> this k-point list depend at all on my choice of NxNxN in (i)). Can I choose >> any the standard 28 k-points quoted in example01? >> >> >> (iii) - How do I actually calculate my band gap from the eigenvalues >> outputted from this 2nd scf? >> >> >> Sorry about my ignorance on this. To compare it with the literature >> band gap, would it also be possible to just use the dos post processing >> tool? I will be needing the dos at some point in the future anyway. Is this >> used in the same way as it is for non-hybrid functionals? >> >> >> Thank you so much for taking the time to read this. It's really >> appreciated. >> >> >> Galvin, >> UCL. >> ------------------------------ >> *From:* Layla Martin-Samos <[email protected]> >> *Sent:* 14 May 2015 13:10 >> >> *To:* PWSCF Forum >> *Subject:* Re: [Pw_forum] Silicon HSE Problem >> >> good! check and see if it is within the litt. values. >> >> cheers >> >> Layla >> >> 2015-05-14 13:05 GMT+02:00 Khara, Galvin <[email protected]>: >> >>> Hey, >>> >>> >>> By using a value of 0.096, (Heyd recommends using a value smaller than >>> 0.15 to reproduce accurate band gaps in semi-conductors), my cutoff >>> energies oscillate as follows; (scf energy, ecut energy). >>> >>> >>> -14.56060124 >>> -14.56579225 -- 20 Ry >>> -14.56576890 >>> -14.56671699 -- 30 Ry >>> -14.56731443 >>> -14.56796231 -- 40 Ry >>> -14.56725086 >>> -14.56720876 -- 50 Ry >>> -14.56751976 >>> -14.56750677 -- 60 Ry >>> -14.56749110 >>> -14.56713377 -- 70 Ry >>> -14.56711227 >>> -14.56709019 -- 80 Ry >>> -14.56706889 >>> -14.56704906 -- 90 Ry >>> -14.56722966 >>> -14.56721482 -- 100 Ry >>> -14.56700415 >>> -14.56699186 -- 110 Ry >>> -14.56697995 >>> -14.56696887 -- 120 Ry >>> -14.56695845 >>> -14.56694874 -- 130 Ry >>> -14.56693973 >>> -14.56693137 -- 140 Ry >>> -14.56692360 >>> -14.56691649 -- 150 Ry >>> -14.56690962 >>> -14.56690322 -- 160 Ry >>> -14.56698048 >>> -14.56697473 -- 170 Ry >>> -14.56696928 >>> -14.56696413 -- 180 Ry >>> -14.56693442 >>> -14.56692978 -- 190 Ry >>> -14.56692543 >>> >>> >>> >>> So the oscillation isn't massive, but I am only using a 2-atom cell. I >>> am basically interested in calculating the energy of my system at elevated >>> electronic temperatures (using femi-dirac smearing), I'm going to go ahead >>> and calculate my band-gap and lattice parameters using this oscillating >>> ecut, and see how close they are to literature. Do you think this is ill >>> advised? >>> >>> >>> Thanks, >>> >>> >>> Galvin >>> ------------------------------ >>> *From:* Layla Martin-Samos <[email protected]> >>> *Sent:* 14 May 2015 09:57 >>> >>> *To:* PWSCF Forum >>> *Subject:* Re: [Pw_forum] Silicon HSE Problem >>> >>> Hi, I think that the safest way is to increase the dimension of your >>> simulation cell (by replicating the unit cell along the three axes) and use >>> the screening param that gives you the correct gap. To have uncontrolled >>> noise may be a source of "random numbers". However, each researcher is >>> responsible for the numbers he publish. The impact of noise depends also on >>> the precision and accuracy that you will need or that you expect to need >>> for further post-processing/analysis/multiscale ... . Personally, I try to >>> have initial steps as good as I can, because for my research a >>> not-that-good initial step has a big and negative impact in the next steps. >>> >>> cheers >>> >>> Layla >>> >>> 2015-05-12 15:19 GMT+02:00 Khara, Galvin <[email protected]>: >>> >>>> Hey, >>>> >>>> >>>> Sorry to bring up an old topic, but from reading the HSE paper, I >>>> need the screening parameter to be between 0.1 and 0.2 for a correct band >>>> gap calculation in silicon. Is it a bad idea to use a value for ecut that >>>> is oscillating (but not by a huge amount)? >>>> >>>> >>>> Thanks, >>>> >>>> >>>> Galvin >>>> ------------------------------ >>>> *From:* Layla Martin-Samos <[email protected]> >>>> *Sent:* 10 April 2015 14:39 >>>> *To:* PWSCF Forum >>>> *Subject:* Re: [Pw_forum] Silicon HSE Problem >>>> >>>> Hi Galvin, maybe the screening_parameter ( HSE has two parameters >>>> exx_fraction and screening_parameter) is not big/small (I never >>>> remember if it is dividing or multiplying) enough for your small cell. >>>> Maybe you could try to check the behavior of the total Energy >>>> convergence with respect to variations of this parameter. (in >>>> principle this parameter control the decay of the short range part of >>>> the coulomb potential in your cell. If the short range part is not >>>> small enough at the borders you may find funny results). >>>> >>>> cheers >>>> >>>> Layla >>>> >>>> 2015-04-10 15:20 UTC+02:00, Khara, Galvin <[email protected]>: >>>> > ?Hey, >>>> > >>>> > >>>> > >>>> > I've been trying to use the HSE hybrid in QE to do some simulations >>>> on Si. >>>> > I'm finding that increasing my ecutwfc is causing the total energy to >>>> > oscillate quite significantly. This seems to indicate a big problem >>>> as this >>>> > should always converge as it is variational.? When I use the plain PBE >>>> > (Si.pbe-hgh.upf) pseudopotential without invoking any hybrid >>>> functional the >>>> > energy converges as expected. >>>> > >>>> > >>>> > It only starts to oscillate when I add the input_dft='hse' >>>> directive, I >>>> > have not added anything non-standard to my input as far as I know (the >>>> > default values for nqx, x_gamma_extrapolation, and exxdiv_treatment? >>>> are >>>> > used). My input file is as follows, >>>> > >>>> > >>>> > " >>>> > >>>> > &control >>>> > prefix='silicon', >>>> > pseudo_dir = '~/pseudo' >>>> > outdir='~/tmp' >>>> > tprnfor = .true. >>>> > tstress = .true. >>>> > / >>>> > &system >>>> > ibrav = 1, celldm(1) =10.18, nat= 8, ntyp= 1, >>>> > ecutwfc = $alat, >>>> > occupations='smearing', >>>> > degauss=0.002, smearing='gaussian', >>>> > input_dft='hse' >>>> > / >>>> > &electrons >>>> > scf_must_converge= .true. >>>> > conv_thr = 1.0e-12 >>>> > mixing_beta = 0.7 >>>> > / >>>> > ATOMIC_SPECIES >>>> > Si 28.0855 Si.pbe-hgh.upf >>>> > >>>> > ATOMIC_POSITIONS crystal >>>> > Si 0.0 0.0 0.0 >>>> > Si 0.5 0.5 0.0 >>>> > Si 0.5 0.0 0.5 >>>> > Si 0.0 0.5 0.5 >>>> > Si 0.25 0.25 0.25 >>>> > Si 0.75 0.75 0.25 >>>> > Si 0.75 0.25 0.75 >>>> > Si 0.25 0.75 0.75 >>>> > >>>> > K_POINTS gamma >>>> > >>>> > >>>> > " >>>> > >>>> > >>>> > Also, I ultimately want to use the HSE hybrid to study Si at elevated >>>> > electronic temperatures (up to 25,000 K) via fermi-dirac broadening, >>>> is this >>>> > unrealistic / unwise with this choice of hybrid? Any help is greatly >>>> > appreciated. >>>> > >>>> > >>>> > >>>> > Thanks, >>>> > >>>> > >>>> > Galvin K, >>>> > Ph.D Researcher, >>>> > University College London >>>> > >>>> >>>> >>>> _______________________________________________ >>>> Pw_forum mailing list >>>> [email protected] >>>> http://pwscf.org/mailman/listinfo/pw_forum >>>> >>> >>> >>> _______________________________________________ >>> Pw_forum mailing list >>> [email protected] >>> http://pwscf.org/mailman/listinfo/pw_forum >>> >> >> >> _______________________________________________ >> Pw_forum mailing list >> [email protected] >> http://pwscf.org/mailman/listinfo/pw_forum >> > > > > -- > -- > Dhirendra > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- -- Dhirendra
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