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?

Thanks in advanced,

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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