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. 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 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) 3) thanks very much for the reference Rev. Mod. Phys. 86, 253 (2014).
regards Seyed Mehrdad Zamzamian Sharif University of Technology, Tehran, Iran Energy engineering department E-mail: [email protected] On Fri, May 31, 2019 at 4:24 AM Kevin May <[email protected]> wrote: > Hi Mehrdad, > > Just a few more thoughts: > > 1) Which reference state are you using for Fe? Jiang et al. use ferromagnetic > BCC Fe in Acta Materialia 56, 3236–3244 (2008). This paper uses constant > volume rather than constant (zero) pressure calculations. I would > definitely do spin polarized calculations. You mentioned your current > pseudopotentials are not recommended for spin polarized calculations--I > would use different ones. > > 2) I'm not sure which pseudopotentials you are using exactly, but I have > had good results using those in the Standard Solid State Pseudopotentials > (SSSP) tables, which comes from very useful work of a team at EPFL ( > https://www.materialscloud.org/discover/sssp/). In your case, this would > be from PSLibrary 0.3.1 PAW for Fe and PSLibrary 1.0.0 PAW for C. Note the > recommended ecutrho is 12*ecutwfc for Fe. The recommended cutoff there is > 90 Ry but you might get away with something smaller if you test it yourself. > > 3) A good reference for defect calculations is Rev. Mod. Phys. 86, 253 > (2014). > > Best, > > Kevin May, PhD > Postdoctoral Associate > Department of Materials Science and Engineering > Massachusetts Institute of Technology > > > > > On Thu, May 30, 2019 at 3:59 AM mehrdad zamzamian < > [email protected]> wrote: > >> Thanks for your attention to my question >> Actually, i defined a supercell (2*2*2), used USPP-PBE.I adjusted the >> cutoff according to what was proposed for it (~70) and for ecut_rho (~400). >> for scf convergence, i set conv=1e-6, ant at here, i set mixing beta~0.04 >> (it helped to reach to convergence fast). and also i set press=0 and its >> threshold to 0.5. I want to calculate the vacancy (lack of Fe) energy. its >> value must be 1.6 eV but i never give the better than 2.2 eV (The >> ridiculous thing is that with molecular dynamics I got a very precise >> amount of 1.6! that of course, its interatomic is more precise than my >> calculation!). according to my PP in this calculation, it is suggested to >> use non-polarized spin. i really do not know what parameter can i changed >> to achieve 1.6 eV. >> In addition, your food for the brain is some hard to be digested! >> >> Regards >> >> Seyed Mehrdad Zamzamian >> Sharif University of Technology, Tehran, Iran >> Energy engineering department >> E-mail: [email protected] >> >> On Wed, May 29, 2019 at 9:48 PM Kevin May <[email protected]> wrote: >> >>> Hi Mehrdad, >>> >>> If you have a large energy difference between the end of the vc-relax >>> algorithm and the final scf step, you may be using a fairly low plane wave >>> cutoff energy for your system, though that may be fine depending on what >>> quantities you are looking at. An important question here is: how are you >>> choosing your convergence criteria? In my experience stresses (vc-relax) >>> require a higher cutoff compared to total energy or forces (just ionic >>> relaxation). >>> >>> This all depends of course on what you are trying to get out of your >>> calculation. In my case I was comparing different magnetic ground states >>> that could be very close in energy, where very small changes in geometry >>> can make a difference. For my specific systems I would converge plane wave >>> cutoff, k-points and convergence threshold with respect to unit cell stress >>> using a reference calculation with very high cutoff, low threshold (10^-9 >>> Ry), and dense k-point mesh. >>> >>> Just some food for thought. I've definitely seen papers in the >>> literature where they claim state A has lower energy than state B. I've >>> reproduced such results using the somewhat lax cutoffs reported, and then >>> found when you actually do CONVERGED calculations, state B is actually >>> lower in energy (whoops!). Convergence is important. >>> >>> Best, >>> >>> Kevin May, PhD >>> Postdoctoral Associate >>> Department of Materials Science and Engineering >>> Massachusetts Institute of Technology >>> >>
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