# How to calculate formation energy using DFT+U?

I noticed that a lot of articles use DFT+U methods to calculate different formation energies when using regular DFT and DFT+U methods. For example, in the attached paper[1] on $$\ce{In2O3}$$, the enthalpy of formation was found to be $$\pu{-9.86 eV}$$ and $$\pu{-10.41 eV}$$ using LDA and LDA+U, respectively.

When calculating the formation energies using DFT+U, I am wondering if the energies of the elemental reference states ($$\ce{In}$$ and $$\ce{O2}$$ in the above case) also need to be calculated using DFT+U, or just from regular DFT, since they often do not have band gaps that DFT+U seeks to correct.

1. Pakpoom Reunchan, Xin Zhou, Sukit Limpijumnong, Anderson Janotti, Chris G. Van de Walle, Vacancy defects in indium oxide: An ab-initio study, Current Applied Physics, Volume 11, Issue 3, Supplement, 2011, Pages S296-S300,DOI: 10.1016/j.cap.2011.03.051

As far as my knowledge, they must be calculated using DFT+U. When you calculate the energy of $$\ce{In}$$ in its bulk state using DFT, it won't lead to the same value using DFT+U.