The optical and magneto-optical (MO) spectra of UGa_(2) are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with generalization of the LSDA+U method which takes into account that in the presence of spin-orbit coupling the occupation matrix of localized electrons becomes nondiagonal in spin indexes. Although, the magnetic moment at the U site of UGa_(2) is described better by LSDA+U than by LSDA, neither the localized LSDA+U model nor the spin-polarized LSDA 5f-itinerant band model can explain all the peculiarities of the experimental MO spectra. The LSDA gives a better description of the optical and MO spectra of UGa_(2) in the 0-1 eV energy interval, but for higher energies the experimental Kerr rotation curve is situated someplace in between the curves obtained by the LSDA and the LSDA+U approximations. The origin of the Kerr rotation in the compound is examined.
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