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The equilibrium geometry of the C 4ν FeOF5 4− center in KMgF3 has been explored by means of density functional theory calculations. Particular attention has been paid to the changes of electronic density on passing from the O h FeF6 3− center in KMgF3 to FeOF5 4−, as well as to the variations undergone by the isotropic superhyperfine constant of axial A s(Fax) and equatorial A s(Feq) fluorine nuclei placed from Fe3+ at distances R ax and R eq, respectively. As a salient feature it is found for FeOF5 4− that, even if R eq = R ax, A s(Feq) is significantly higher than A s(Fax) thus pointing out that such quantities depend not only on the Fe3+–F− distance of the corresponding bond. Accordingly the obtained equilibrium distances R ax = 0.2178 nm and R eq = 0.2055 nm are not reproduced by any model based on the bond transferability. A slight off-center 0.018 nm displacement of Fe3+ towards O2− is derived through the present calculations which reproduce reasonably well the experimental ratio A s(Feq)/A s(Fax) = 2.3.