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
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