<div dir="ltr"><div><div><div><div>Dear Prof. Nicola Marzari,<br><br></div>Thank you so much for your suggestions. <br><br></div>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) <br><br>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)<br><br>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) .<br><br>These calculations are performed with the same pseudo-potentials.<br><br>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.<br><br></div>Thanks<br><br></div>CMI Okoye <br></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div><div><div><i>------------------------------<br>Chukwuemeka M I <b>Okoye</b><br></i></div><i>Department of Physics and Astronomy,<br></i></div><i>University of Nigeria, <br></i></div><i>Nsukka, Enugu State,<br></i></div><i>Nigeria</i><br><div>Telephone: +234 7038766990<br></div><div>E-mail: <a href="mailto:cmi.okoye@unn.edu.ng" target="_blank">cmi.okoye@unn.edu.ng</a><br></div></div></div></div></div></div></div></div>
<br><div class="gmail_quote">On Wed, Jul 5, 2017 at 11:41 PM, Nicola Marzari <span dir="ltr"><<a href="mailto:nicola.marzari@epfl.ch" target="_blank">nicola.marzari@epfl.ch</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
<br>
Dear Chukwuemeka,<br>
<br>
there are two possibilities:<br>
<br>
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<br>
multiple of these in a supercell) you just need to compare<br>
the energies out of PWscf for different arrangements; in<br>
each case, you want to calculate for the particular arrangement you<br>
chose the energy at relaxed atomic positions, and relaxed unit cell<br>
vectors. This would allow you to identify the best candidate among your<br>
trials (but of course you wouldn't know if there are better candidates,<br>
or if the system is thermodynamically stable with respect to decomposition in other, simpler compounds).<br>
<br>
2) if you have a more complex case, where you are comparing compounds with different stoichiometries, you want to compare formation energies.<br>
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<br>
be the diamond lattice at equilibrium). So, for each of the 4 elements<br>
you need the energies of their elemental ground-state structure per atom<br>
(for Si, you divide by 2, since you have 2 atoms in the unit cell), and<br>
then you want to compare formation energies defined as<br>
<br>
E(A_i,B_j,C_k,D_l)-i*E(A)-j*E(<wbr>B)-k*E(C)-l*E(D)<br>
<br>
where E(A_i,B_j,C_k,D_l) is the energy of your compound (at equilibrium)<br>
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<br>
to decomposition in the end compounds - but you still wouldn't know<br>
if it is stable or not with respect to say decomposing into different<br>
binary compounds, or combinations of unaries, binaries, and ternaries.<br>
<br>
For more in-depth understanding, have a look at:<br>
<a href="http://oqmd.org/analysis/gclp/" rel="noreferrer" target="_blank">http://oqmd.org/analysis/gclp/</a><br>
<br>
nicola<span class=""><br>
<br>
<br>
On 05/07/2017 23:59, Chukwuemeka Okoye wrote:<br>
</span><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">
Dear All,<br>
<br>
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?<br>
Thanks.<br>
<br>
CMI Okoye<br></span>
/-----------------------------<wbr>-<br>
Chukwuemeka M I *Okoye*<br>
/<br>
/Department of Physics and Astronomy,<br>
/<br>
/University of Nigeria,<br>
/<br>
/Nsukka, Enugu State,<br>
/<br>
/Nigeria/<br>
Telephone: +234 7038766990<br>
E-mail: <a href="mailto:cmi.okoye@unn.edu.ng" target="_blank">cmi.okoye@unn.edu.ng</a> <mailto:<a href="mailto:cmi.okoye@unn.edu.ng" target="_blank">cmi.okoye@unn.edu.ng</a>><br>
<br>
<br>
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<br>
-- <br>
------------------------------<wbr>------------------------------<wbr>----------<br>
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>
Director, National Centre for Competence in Research NCCR MARVEL, EPFL<br>
<a href="http://theossrv1.epfl.ch/Main/Contact" rel="noreferrer" target="_blank">http://theossrv1.epfl.ch/Main/<wbr>Contact</a> <a href="http://nccr-marvel.ch/en/project" rel="noreferrer" target="_blank">http://nccr-marvel.ch/en/proje<wbr>ct</a><br>
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