[Pw_forum] Comparison of total energies

Chukwuemeka Okoye cmi.okoye at unn.edu.ng
Thu Jul 6 16:53:32 CEST 2017

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.


CMI Okoye

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

*Department of Physics and Astronomy,*

*University of Nigeria, *

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

On Wed, Jul 5, 2017 at 11:41 PM, Nicola Marzari <nicola.marzari at epfl.ch>

> Dear Chukwuemeka,
> there are two possibilities:
> 1) if you are comparing the same stoichiometries, but different
> arrangements (e.g. 1 A atom, 1 B atom, 1 C atom, 1 D atom, or a
> multiple of these in a supercell) you just need to compare
> the energies out of PWscf for different arrangements; in
> each case, you want to calculate for the particular arrangement you
> chose the energy at relaxed atomic positions, and relaxed unit cell
> vectors. This would allow you to identify the best candidate among your
> trials (but of course you wouldn't know if there are better candidates,
> or if the system is thermodynamically stable with respect to decomposition
> in other, simpler compounds).
> 2) if you have a more complex case, where you are comparing compounds with
> different stoichiometries, you want to compare formation energies.
> For that, you could e.g. choose for each element its energy per atom at
> the ground-state structure of the elemental system (eg for Si, it would
> be the diamond lattice at equilibrium). So, for each of the 4 elements
> you need the energies of their elemental ground-state structure per atom
> (for Si, you divide by 2, since you have 2 atoms in the unit cell), and
> then you want to compare formation energies defined as
> E(A_i,B_j,C_k,D_l)-i*E(A)-j*E(B)-k*E(C)-l*E(D)
> where E(A_i,B_j,C_k,D_l) is the energy of your compound (at equilibrium)
> with an integer number i of atoms A, j atoms B, k atoms C, and l atoms D,
> and E(A) is the energy of the elemental compound for A at equilibrium,
> divided by the number of atoms that the elemental compound A as in the unit
> cell (same for E(B), ....). If the formation energy is negative, you would
> know that your system is stable, at 0K, with respect
> to decomposition in the end compounds - but you still wouldn't know
> if it is stable or not with respect to say decomposing into different
> binary compounds, or combinations of unaries, binaries, and ternaries.
> For more in-depth understanding, have a look at:
> http://oqmd.org/analysis/gclp/
>                         nicola
> On 05/07/2017 23:59, Chukwuemeka Okoye wrote:
>> Dear All,
>> I am using Quantum Espresso to perform calculations on quaternary Heusler
>> compounds. My question is, can I compare the total energies of different
>> types of atom arrangement in other to determine the most stable
>> configuration(correct ground state) between Type-I, Type-II and Type-III?
>> Thanks.
>> 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 <mailto:cmi.okoye at unn.edu.ng>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
> --
> ----------------------------------------------------------------------
> 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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