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.