I think you need to consider what your reference states are. If you are interested in the enthalpy of formation of the doped system from its constituent elements, then your suggested method 2. should work. However, in practice, this is not normally how we tend to think about doping. However, if you are interested in investigating the reaction of undoped LaVO4 to give the doped system, which I think might be more relevant, then something more like your suggested method 1. could work. However, it may also be possible that one can introduce Eu into the system in the form of a compound, rather than as elemental Eu, and essentially there could be a displacement type reaction. For example, it's not inconceivable that one could react a Eu oxide with LaVO4, forming your doped system, and displacing La in the form of an oxide.

I don't know much about lanthanide chemistry, so I can't make any firm suggestions. However, what I can say is that it is necessary to consider where Eu is coming from. The enthalpy of the doping reaction will depend on the relative stabilities of the doped system, LaVO4 and the Eu compound, so I think perhaps you should look in the literature to see if any experimental studies have tried this kind of doping and see what they used as their Eu source as a reactant.

I hope this helps.

Thank you Michael David Higham for your reply. I will look such type of experimental studies.

Dear Shilendra,

Unlike semi-empirical methods (AM1, PM3 and etc.) DFT does much better on relative energies than on absolute heats of formation. It is easier to get the experimental heat of formation of a few compounds, and then compute the reaction energy between those and yours. Then you compute the heat of formation of yours based on the differences.

Hoping this will be helpful,

Rafik