  We report on an ab initio strategy based on Density Functional Theory to
identify the muon sites. Two issues must be carefully addressed, muon
delocalization about candidate interstitial sites and local structural
relaxation of the atomic positions due to $\mu^+$-sample interaction. Here, we
report on the validation of our strategy on two wide band gap materials, LiF
and YF3, where localization issues are important because of the interplay
between muon localization and lattice relaxation
