[Wien] Formation energy of impurity charge states

Fri Apr 12 14:02:13 CEST 2019

What you are trying to do is generate an estimate of the energy dV*Q where
dV is the potential offset, and Q is your charge. The atoms by the charged
defect will not be close to the same as the bulk, you want one which is far
enough away that it is a viable reference state. You then compare this to
your reference material, which does not have to be a supercell and could be
just bulk. (Sometimes it is easier to ensure that k-points and other things
are identical by using a supercell without the defect.)

Be aware that Wien2k does not attempt to do any of the charged cell
corrections that are in the literature. For these you are on your own! In
principle you want to extrapolate with a series of cells of increasing size
in order to calculate the defect-defect coupling energy as a function of
cell size. And/or calculate the correction. I remember many years ago
looking at this with Peter, and I am not sure either of us was terrible
convinced by the corrections in the literature.

Also, be aware that the approach of charged cells assumes that there is no
interaction between, for instance, holes and the defect. I recently did a
(neutral) calculation of a Ni vacancy in NiO. What I expected to find was
two Ni3+, but instead I found a couple of delocalized hole on O sites near
the vacancy. (I still need to ponder whether I believe this.) A charged
cell calculation where one adds two electrons at the same time as one
creates a Ni vacancy will not include the electrostatic interaction between
the (negative) vacancy and the delocalized hole.

Also, there is a recent paper by Pantelides in PRL (with some later
comments by others) where he disagrees with the conventional approach. I do
not know who is right.

On Fri, Apr 12, 2019 at 12:34 PM Marcelo Barbosa <
marcelo.b.barbosa at gmail.com> wrote:

> Dear Prof Laurence,
>
> First of all, thank you for stepping into the discussion and for
> clarifying the difference between a pseudo-potential calculation and an
> all-electron code in this situation.
>
> Can I then use the core energies obtained using for example "grep :1S
> case.scf”?
>
> By the way, I have another question…
> This correction is only needed for a charged cell and, as far as I
> understood, for the method performed in the case of pseudo-potential
> calculations the alignment must be done between the charged supercell and a
> supercell of the same size from the bulk material.
> However, for the procedure you described, the core energy difference
> should be calculated between the charged supercell and the same supercell
> in the non-charged state, correct?
>
> Best regards,
> Marcelo
>
> On 11 Apr 2019, at 19:19, Laurence Marks <L-marks at northwestern.edu> wrote:
>
> I think this conversation has gone in an incorrect direction. What you are
> trying to do is align the relative energies/potential as there is an offset
> due to subtraction of the mean inner potential (to avoid singularities)
> that is different for charged and non-charged cells.
>
> With a pseudo-potential calculation, the electrostatic potential is
> readily available so people use it to align.
>
> With an all-electron code the electrostatic potential can be generated,
> but there are far easier methods to align! All you need to do is compare
> the core energies of atoms well away from the defect. This gives you the
> relevant energy axis shift.
>
> Then you have to make whatever correction you trust....which is not a
> trivial issue.
>
> On Thu, Apr 11, 2019 at 11:34 AM Marcelo Barbosa <
> marcelo.b.barbosa at gmail.com> wrote:
>
>> Thank you very much for your help!
>>
>> Best regards,
>> Marcelo
>>
>> On 11 Apr 2019, at 16:16, SM Alay-e-Abbas <alayabbas at gmail.com> wrote:
>>
>> --> Since I need to calculate the electrostatic potential, is following
>> method the correct approach? Is it also correct for spin polarized
>> calculations?
>> *For electrostatic potentials you should be using case.vcoul. I don't
>> think that the magnetic order matters here since there is only one output
>> for electrostatic potential (*.vcoul). *
>>
>> --> Create the case.in5 file with the correct plane and number of points
>> to be calculated
>> *and s**et ny = 1!*
>>
>> --> Use “x lapw5 -d” to create the file lapw5.def and then change the
>> unit 9 from “case.clmval” to “case.vtotal”
>> *for electrostatic potentials, it should be case.vcoul*.
>>
>> --> Run lapw5 and get the values in case.rho
>> *The results you should be looking for are in case.rho_onedim*
>>
>> On Thu, Apr 11, 2019 at 12:09 PM Marcelo Barbosa <
>> marcelo.b.barbosa at gmail.com> wrote:
>>
>>> Dear Alay,
>>>
>>> Thank you very much for your help!
>>>
>>> I am able to use your method to select the appropriate plane and the
>>> number of points in that plane to be calculated using lapw5.
>>> I just not entirely sure that I correctly understood the rest of the
>>> procedure by reading the user’s guide.
>>>
>>> Since I need to calculate the electrostatic potential, is following
>>> method the correct approach? Is it also correct for spin polarized
>>> calculations?
>>>
>>> - Create the case.in5 file with the correct plane and number of points
>>> to be calculated
>>> - Set “iuntits” to ATU in case.in5 to get the values in Ry
>>> - Set “cnorm” to VAL in case.in5
>>> - Use “x lapw5 -d” to create the file lapw5.def and then change the unit
>>> 9 from “case.clmval” to “case.vtotal”
>>> - Run lapw5 and get the values in case.rho
>>>
>>> Best regards,
>>> Marcelo
>>>
>>>
>>> On 4 Apr 2019, at 21:30, SM Alay-e-Abbas <alayabbas at gmail.com> wrote:
>>>
>>> Hello Marcelo,
>>>
>>> You may do this by selecting an appropriate atom centered plane (with
>>> reasonable width) and then setting npy = 1 in case.in5 before running
>>> lapw5. See section 8.13.3 of the userguide for more details.
>>>
>>> Best Regards,
>>>
>>> Alay
>>>
>>> On Thu, Apr 4, 2019 at 5:40 PM Marcelo Barbosa <
>>> marcelo.b.barbosa at gmail.com> wrote:
>>>
>>>> Dear Sirs,
>>>>
>>>> When comparing the formation energies of two charge states of an
>>>> impurity in a semiconductor as a function of the Fermi level, it is said
>>>> that a correction term must be added to align the electrostatic potential
>>>> from the supercell with the impurity (using a position far from the
>>>> impurity) and from a supercell of the pure bulk material with the same size
>>>> (see e.g. DOI: 10.1063/1.1682673).
>>>>
>>>> The electrostatic potential is calculated in LAPW0, but how can I find
>>>> the value corresponding to a specific position in my supercell?
>>>>
>>>>
>>>> Best regards,
>>>> Marcelo
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>
> --
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to think what nobody
> else has thought", Albert Szent-Gyorgi
> www.numis.northwestern.edu ; Corrosion in 4D:
> MURI4D.numis.northwestern.edu <http://muri4d.numis.northwestern.edu/>
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-- 
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
Co-Editor, Acta Cryst A
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