I am trying to calculate the formation energy of $\ce{Mg2Si}$ and for the calculation of lowest energy configuration of Bulk $\ce{Si}$: $E(\ce{Si})$ and bulk Mg $E(\ce{Mg})$. I have opted for both structures from the Materials Project with the same space group Fm3m. I am wondering that if I change the space group of both i.e by employing different structures from another space group formation energy changes drastically. For the same space group, the value of formation energy does not match with previous results.

How will I decide the correct prediction of bulk materials (Mg, Si)?

  • $\begingroup$ +1 but please look at my edit and make the same edits for the rest of the question. We have standards here for formatting and if you don't meet those standards, others have to spend time bringing your question up to those standards, which is unfair for them. $\endgroup$ Commented Mar 23, 2021 at 20:49
  • $\begingroup$ @Shalini Have you tried to subtract energy of single atom mg and si? Single atom means large box with one atom $\endgroup$ Commented Mar 24, 2021 at 1:11
  • $\begingroup$ @Jack Thanks for the edits. Did you see my comment though? This user has been told in previous comments (for example here: mattermodeling.stackexchange.com/questions/4338/…) to make note of people's edits to use ChemJax, and if we keep doing it for them I worry that we might not see any improvement from the user herself! $\endgroup$ Commented Mar 24, 2021 at 3:10
  • $\begingroup$ @Jack please see this: mattermodeling.meta.stackexchange.com/q/245/5 $\endgroup$ Commented Mar 24, 2021 at 3:45
  • $\begingroup$ @NikeDattani Sorry for that. Next time I will follow the rules. $\endgroup$
    – Jack
    Commented Mar 24, 2021 at 5:25

1 Answer 1


Basically, for obtaining the formation energy we need to calculate the energy of the individual phases at their ground state structure. For example in your case you have to consider Si and Mg in their ground state i.e. diamond cubic (Fd-3m) and hcp (P63/mmc), respectively.

Hope it helps.

  • $\begingroup$ Yes this is what I am asking , you suggested different space groups for both Fd-3m & P63/mmc . So this is not necessary to take structure from same space group ? $\endgroup$
    – Shalini
    Commented Mar 24, 2021 at 10:26
  • $\begingroup$ You want to take the most stable structure. The formation energy can be thought of as "you have these two materials, what energy is released / absorbed in the process of rearranging the atoms". For oxides for example, you may need to use oxygen as a gas phase. $\endgroup$ Commented Mar 24, 2021 at 14:38
  • $\begingroup$ @ Shalini, Yes, it is not mandatory to consider the same space group always. $\endgroup$ Commented Mar 25, 2021 at 11:52

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