In mp database, I found the formation energy for water molecules(mp-697111) should be -1.393 eV*3=-4.179/formula.
However, if I search the Final Energy/Atom for H2O(mp-697111), H2(mp-632291) and O2(mp-1091399), each of them is with the smallest e above hull and green highlight. Then I calculated the formation energy for H2O by hands:

Where does the error come from? Similarly, I calculated the formation energy for Mg(OH)2, I found there is also a deviation between the data in MP and data calculated by hands.
This troubles me for a long time if you have any idea, please leave a message， Thanks in advance.

Thanks for your reply!
I know formation energies are in eV/Atom and I calculated the formation energy for the water molecule, -1.393*3 = -4.179 eV/formula.
Mg question is If I search the energy of H2,O2 and H2O in mp database, then I calculate the formation energy by hands(the answer is -3.1765/formula), where this deviation(between -4.179 and -3.1765) come from?

Hello, when i use the corrected energy and your equation, I calculated the formation energy by hands , then it have the result is -135.2761 eV. Can you tell me what my problem? Thank you so much

I do not understand the formation energy calculation. EH2O (corrected energy) is -62.3130 eV. (-62.313+6.78+0.5*9.857) = -50.6045 eV. Dividing it by 3 does not produce the result. Please help me to understand.