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

[Kevin May]

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 <
mehrdad.zamzamian at gmail.com> 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 <kmay at mit.edu> wrote:
>
>> 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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