I am studying of double peroskites and trying to find its magnetic and electrical properties usinf VASP. Please help me to find energy difference per chemical formula between ferromagnetic and anti ferromagnetic .
Dear Rajesh Pathak,
I will try to help.
If we have structure FM ABO3 (its chemical formula has 5 atoms) and has 10 atoms in its unit cell structure (A2B2O6), the energy per chemical formula (E per formula1) = 1/2 * the total energy from DFT for FM. It is also applied for AFM one: E per formula2
Then, the energy difference per chemical formula would be E per formula2-E per formula1.
This may help.
Muhammady
JP
Dear Rajesh Pathak
before doing the suggested calculations, check what is the space group of the structure that you are using to perform the calculations. Also check the total number of nonequivalent atoms.
If the two materials being compared have the same stoichiometry but differ in geometric structure the comparison is relatively easy. For each material, compute the total energy for a unit cell, then divide the total energy by the number of formula units in the unit cell. The numbers are directly comparable. Keep in mind that the total energies themselves depend on the definition of the zero of energy used by the electronic structure code. The energy difference is the physically relevant quantity. (The zero of energy is typically taken as corresponding to the energy cost of moving all of the electrons present to infinite separation, and then separating all of the atomic nuclei to infinite separation. Alternatively, in the case of a pseudopotential calculation, it corresponds to moving all of the valence electrons to infinity and then separating all of the atomic cores.)
If the two materials have the same geometric structure but differ in electronic structure, then the problem is much more difficult. Effectively, one (or possibly both) of the structures is an electronically excited state. Computation of the energy will require a theoretical method suitable for excited states, such as constrained DFT. If the unit cells are geometrically identical, the engines are directly comparable.
If the two materials differ in stoichiometry, then the energies are not directly comparable. One needs to add the energy of a third material to match the stoichiometry. For example. Comparing Fe2O3 and Fe3O4, the latter is Fe rich. A possible comparison is 4{Fe2O3} + 3{Fe} 3{Fe3O4}. One computes the total energy for Fe2O3 and for elemental Fe. The sum of an appropriate number of formula units of each is compared to the total energy for Fe3O4. In this case the energy difference is dependent on the choice of third material so a physically meaningful choice is essential.
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