# Section 'Calculate reaction'

Hello everybody,

I have a question about the way to calculate the enthalpy in the section ‘Calculate reaction’. How is it calculate by mp?

I have this reaction:
Li2SO4 + 2MnSO4 → Li2Mn2(SO4)3
and, while doing DFT I got the same final energies per atom that I found in mp for each compound. But…If I calculate the enthalpy of the reaction using these energies I get an enthalpy 12 units bigger than the one that I got in the section ‘Calculate reaction’.

Can you help me to understand how do I get such a different result?

Iciar

Hi @Iciar_ms, would you mind posting the steps you used to calculate your result for reaction enthalpy?

If you are using energies per atom in your calculation, note that you need to multiply by the number of atoms in each formula first to get energies/f.u. Then when you multiply by the stoichiometric coefficients, you will get the enthalpy of reaction in eV (per mol of reaction).

I tried this with the reaction you posted and I get the same answer as the Reaction Calculator on the Materials Project website (i.e., total energy of products - total energy of reactants):

\Delta H_{rxn} =(1*19*-2.261) - ((1*7*-2.415) + (2*6*-2.167)) = -0.05 eV

Hello @mattmcdermott thank you so much for your answering. I haven’t notice that I had to multiply by the number of atoms in each formula.

In any case with my calculations I get a result comparable with the final energy per atom but different from the formation energy per atom.

So, with my data I would have (using the data from mp):
ΔH = (1 * 19 * -6.47) - ((1 * 7 * -5.77) + (2 * 6 * -6.82) = -0.7eV

Which would be the difference between them?

Thank you so much, it has been very useful for me.

You’re welcome @Iciar_ms ! Yes, if you use the final DFT energies that MP reports under the Calculation Summary section (i.e., not formation energies), then you will also get the same answer of around -0.05eV:

\Delta H_{rxn} =(1*19*-6.4797) - ((1*7*-5.8825) + (2*6*-6.8236)) = -0.0536 eV

In the equation you posted, the energy of MnSO4 is different than what is on MP (-5.77 vs -5.88), this would explain why you get a different number.

Thank you so much @mattmcdermott , you solved all my doubts!

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