We have obtained analytical expressions for effective parameters of the crystal field that acts on spin-orbit multiplets of 4f~N configurations taking into account admixture to them of 4f~(N-1)5d excited states and ligand-to-metal charge-transfer states. As an example, we analyze splittings of the ground and excited multiplets of Pr~(3+) and Tm~(2+) ions in some crystals without an inversion center. The effect of mixing of states of different configurations is most strongly pronounced for the ~1G_4 and ~1D_2 excited multiplets. The interconfigurational contribution to splittings is different for different multiplets. This circumstance makes it possible to estimate the values of the parameters of the odd-symmetry crystal field, which causes mixing of the 4f~N and 4f~(N-1)5d states, and the covalence parameters of rare-earth ion-ligand bonds.
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