# Formation energy calculation

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)?

• +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. Mar 23 at 20:49
• @Shalini Have you tried to subtract energy of single atom mg and si? Single atom means large box with one atom Mar 24 at 1:11
• @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! Mar 24 at 3:10
• @Jack please see this: mattermodeling.meta.stackexchange.com/q/245/5 Mar 24 at 3:45
• @NikeDattani Sorry for that. Next time I will follow the rules.
– Jack
Mar 24 at 5:25