Hi Mehrdad, I forgot that you could just set ibrav = 3 instead of using the conventional BCC unit cell. That way you can just use one atom in the calculation. Also, don't forget to set a non-zero starting magnetization and change ecutrho to be 1080. If you make these 3 adjustments to your previous input file for the bulk BCC Fe it should work.
I did a quick calculation of bulk BCC Fe on my laptop and got 329.27274577 Ry. Using your supercell energies I got a vacancy formation energy of 1.69 eV. Pretty close to the 1.66 eV reported by Jiang et al. that your reference uses. They used PW91-GGA, different PAW datasets and different software so I think that's pretty good. Double-check for yourself though, of course. Best, Kevin May, PhD Postdoctoral Associate Department of Materials Science and Engineering Massachusetts Institute of Technology On Tue, Jun 18, 2019 at 10:30 PM Kevin May <[email protected]> wrote: > Hi Mehrdad, > > So you are indeed using an isolated Fe atom as your reference state. That > is not the conventional chemical potential for elemental iron. To have a > fair comparison to the calculated value the literature, please try a > vc-relax calculation of ferromagnetic BCC iron (like this CIF file: > https://www.crystallography.net/cod/9008536.cif). Divide the energy by 2 > (since there are two atoms per unit cell) to get the chemical potential > reference for iron. See my comment from the last email: > > By* E(single Fe) you mean 1/2 the energy of ferromagnetic BCC Fe (2 Fe >> per unit cell), rather than an isolated Fe atom, right? That is the >> chemical potential reference for Fe that was used in calculating the 1.6 eV >> formation energy value.* And for what it's worth I don't think point >> defect calculations are that trivial. >> >>> 3) thanks very much for the reference Rev. Mod. Phys. 86, 253 (2014). >>> >> >> Regarding chemical potential reference, see *section II.B.2 of this >> article*. >> > > Best, > Kevin May, PhD > Postdoctoral Associate > Department of Materials Science and Engineering > Massachusetts Institute of Technology > > > > > > On Tue, Jun 18, 2019 at 11:54 AM mehrdad zamzamian < > [email protected]> wrote: > >> Dear Kevin >> Hi again >> Sorry i bothered you many times for asking many questions, but i am >> really confused about my case >> After using the paw-pp that you suggested, i obtained E(f-vacancy)~2.0 >> eV!! here in attachment, i gave my input. i used a 128-atom supercell for a >> perfect crystal (E=32199.76950786 Ry) and by removing a Fe atom, i had >> E=31870.37251766 Ry and isolated Fe E=329.18792433 Ry. It seems that i >> cannot obtain a better result than 2 eV. I appreciated if you give me any >> suggestions. the paper that you mentioned talked about "Ecorr". what >> exactly is this parameter? >> >> On Sat, Jun 1, 2019 at 11:27 PM Kevin May <[email protected]> wrote: >> >>> Hi Mehrdad, >>> >>> On Sat, Jun 1, 2019 at 1:58 PM mehrdad zamzamian < >>> [email protected]> wrote: >>> >>>> With regard >>>> Dear Kevin >>>> 1) Actually, my reference is computational material science 44(2008) >>>> 690-694 (although they reported delta(E)= -0.456 eV/atom that i don't know >>>> what it is. because i said that it must be 1.6 eV according to >>>> https://doi.org/10.1080/09506608.2018.1560984). i also used >>>> spin-polarized (0.2 for Fe) but the same result was obtained (not better >>>> than 2.2 eV). I also used constant volume (by using relax calculation not >>>> vc_relax), but i had the same results. >>>> >>> >>> The first paper you mention does not calculate vacancy formation energy, >>> the -0.456 eV/atom is the bulk formation energy of orthorhombic Fe3C from >>> GGA. The second paper is a review article that cites the paper by Jiang et >>> al. that I mentioned in my last email for their ~1.6 eV formation energy of >>> a Fe vacancy. >>> >>>> 2) thanks for the proposed links, but in that reference they offered >>>> two non-consistent pps (C.pbe-n-kjpaw_psl.1.0.0.UPF >>>> and Fe.pbe-spn-kjpaw_psl.0.2.1.UPF) that i cannot use for Fe3C. i used >>>> these pp: >>>> C.pbe-n-rrkjus_psl.1.0.0.UPF >>>> Fe.pbe-spn-rrkjus_psl.1.0.0.UPF >>>> >>> >>> I'm not sure what you mean by "non-consistent", or why you can't use >>> those PAW datasets for your calculation. If you need to use ultrasoft for >>> another reason, I'd still recommend using Fe.pbe-spn-rrkjus_psl.0.2.1.UPF >>> instead of the psl.1.0.0 version. >>> >>> I am really confused about how can i calculate this rather simple >>>> parameter. i should mention that i calculate the Fe vacancy with: >>>> E(Fe-vacancy)=E(perfect Fe3C)- E(Fe3C with lack of one Fe atom)- >>>> E(single Fe) >>>> >>> >>> By E(single Fe) you mean 1/2 the energy of ferromagnetic BCC Fe (2 Fe >>> per unit cell), rather than an isolated Fe atom, right? That is the >>> chemical potential reference for Fe that was used in calculating the 1.6 eV >>> formation energy value. And for what it's worth I don't think point defect >>> calculations are that trivial. >>> >>>> 3) thanks very much for the reference Rev. Mod. Phys. 86, 253 (2014). >>>> >>> >>> Regarding chemical potential reference, see section II.B.2 of this >>> article. >>> >>> >>>> regards >>>> >>>> Seyed Mehrdad Zamzamian >>>> Sharif University of Technology, Tehran, Iran >>>> Energy engineering department >>>> E-mail: [email protected] >>>> >>> Good luck, >>> >>> Kevin May, PhD >>> Postdoctoral Associate >>> Department of Materials Science and Engineering >>> Massachusetts Institute of Technology >>> >>>
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