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

[Kevin May]

Hi Mehrdad,

On Sat, Jun 1, 2019 at 1:58 PM mehrdad zamzamian <
mehrdad.zamzamian at gmail.com> 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: mehrdad.zamzamian at gmail.com
>
Good luck,

Kevin May, PhD
Postdoctoral Associate
Department of Materials Science and Engineering
Massachusetts Institute of Technology
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