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](https://doi.org/10.1016/j.cap.2011.03.051.)