Abstract: A quantum-chemical model of the minimal cluster in xenotime has been proposedtaking into account the ionic-crystal medium potential. The[YO₈]^5– cluster includesoxygen atoms from six phosphate anions. Owing to the breaking of the covalentP–O bonds, each oxygen atom contributes one unpaired electron to the cluster.The spin state is characterized by a zero ‹Ŝ› vector and a doubled dispersionD = 2 (‹Ŝ²› –‹Ŝ›²) = 7.98 a. u. Structure of the[YO₈]^5– cluster does notfavour the replacement of yttrium(III) with thorium(III) or uranium(III). Thesimulations have been performed via the DFT method using the (U)PBE0functional.