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The energy band structure, as well as partial and total densities of states have been calcu-
lated for LaF3:Yb and LaF3:Lu crystals within density functional theory using the projector augmented
wave method and Hubbard corrections (DFT + U). The influence of geometric optimization on the
results of energy band calculations of LaF3:Ln crystals (Ln = Yb, Lu) was analysed and the absence
of relaxation procedure is confirmed to negatively influence the energy position of states, and the
variability between obtained results of different optimization algorithms are within the calculation
accuracy. The top of the valence band of LaF3 is confirmed to be formed by the 2pF--states and the
bottom of the conduction band is formed by the 5d-states of La3+. The positions of the 4f-states and
5d-states of activator ions in LaF3 were studied. It is shown that the 4f-states of Yb3+ are slightly
above the top of the valence band and the 4f-states of Lu3+ to be 3.5 eV below the top of the valence
band. The energy levels of the 5d states of the impurities are energetically close to the bottom of the
LaF3 conduction band. The calculated band gap of 9.6 eV for LaF3 is in a good agreement with the
experimental result and is not affected by impurity ions.