[QE-users] higher total energy in the last stage of vc-relax

[mehrdad zamzamian]

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: mehrdad.zamzamian at gmail.com

On Fri, May 31, 2019 at 4:24 AM Kevin May <kmay at mit.edu> 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 <
> mehrdad.zamzamian at gmail.com> 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: mehrdad.zamzamian at gmail.com
>>
>> On Wed, May 29, 2019 at 9:48 PM Kevin May <kmay at mit.edu> 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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