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 CMI Okoye *------------------------------Chukwuemeka M I Okoye* *Department of Physics and Astronomy,* *University of Nigeria, * *Nsukka, Enugu State,* *Nigeria* Telephone: +234 7038766990 E-mail: [email protected] On Wed, Jul 5, 2017 at 11:41 PM, Nicola Marzari <[email protected]> wrote: > > > 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: [email protected] <mailto:[email protected]> >> >> >> _______________________________________________ >> Pw_forum mailing list >> [email protected] >> 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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