[Pw_forum] Comparison of total energies

[Chukwuemeka Okoye]

Dear Prof. Marzari,
Thanks a lot for your invaluable response.
Regards
CMI Okoye



*------------------------------Chukwuemeka M I Okoye*

*Department of Physics and Astronomy,*

*University of Nigeria, *

*Nsukka, Enugu State,*
*Nigeria*
Telephone: +234 7038766990
E-mail: cmi.okoye at unn.edu.ng

On Thu, Jul 6, 2017 at 4:19 PM, Nicola Marzari <nicola.marzari at epfl.ch>
wrote:

> On 06/07/2017 16:53, Chukwuemeka Okoye wrote:
>
>> Dear Prof. Nicola Marzari,
>>
>> Thank you so much for your suggestions.
>>
>> At the moment, I am actually interested in comparing the same 1: 1: 1: 1
>> stoichiometry for the quaternary compound, but different  atom arrangements
>> e.g.Type-I: 1 A atom(0,0,0), 1 B atom(.5,.5,.5), 1 C atom(0.25,0.25,0.25),
>> 1 D atom(0.75,0.75,0.75)
>>
>> Type-II: 1 A atom(0,0,0), 1 B atom(0.25,0.25,0.25), 1 C
>> atom(0.5,0.5,0.5), 1 D atom(0.75,0.75,0.75)
>>
>> Type-III: 1 A atom(0,0,0), 1 B atom(0.25,0.25,0.25), 1 C
>> atom(0.75,0.75,0.75), 1 D atom(0.5,0.5,0.5) .
>>
>> These calculations are performed with the same pseudo-potentials.
>>
>> Really, my worry is on the relevance of the total energies in determining
>> the most stable configuration of atomic arrangement since Quantum Espresso
>> is not an all-electron method.
>>
>> Thanks
>>
>
>
> I think you'll be fine, but read the literature I sent. You raise
> several questions?
>
> 1) is QE able to reproduce all-electron
> results. The answer is yes, broadly speaking, and especially I would say
> for relative changes in the formation energies (rather than the absolute
> ones). Not to mention that all-electron results with one code do not
> agree perfectly with another code - so there is no truth. Read:
> https://molmod.ugent.be/deltacodesdft
>
> 2) is PBE, or PBESOL, or whatever xc potential, good enough for the
> thermodynamics of solids? Mostly yes, but not always. But typically good
> enough for your problem, since you are asking relative rankings of very
> similar systems.
>
> 3) are your chosen pseudopotentials good? my suggestion is for
> http://materialscloud.org/sssp, remembering to cite who made the
> original pseudos
>
> 4) remember also that you are calculating total energies at 0K; would the
> free energies at 500K be ordered in the same way? probably yes,
> for your case, but it's relevant for systems where e.g. configurational
> entropy is large (alloys).
>
> nicola
>
>
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
>
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