I am trying to describe the 4f and 5d energy level of Eu2+(Ce3+) ions in the compounds. In my work, I have a problem about the 4f state of Eu2+(Ce3+).
The usual way to localize the 4f state in the band gap of host is the "DFT+U" method. For the determined the "U" value, we need the corresponding experimental value, such as the 4f-VBM(the maximum of valence band).
So, if we don't have such experimental information, how we can determine the "U" value in our calculation?