Calculation of magnetic hyperfine constants

The magnetic hyperfine constants of the V sub(K) center in CaF2, SrF2 and BaF2 have been calculated assuming a phenomenological model, based on the F2- 'central molucule', to describe the wavefunction of the defect. Calculations have shown that introduction of a small degree of covalence, between this central molecule and neighboring ions, is necessary to improve the electronic structure description of the defect. It was also shown that the results for the hyperfine constants are strongly dependent on the relaxations of the ions neighboring the central molecule; these relaxations have been determined by fitting the experimental data. The present results are compared with other previous calculations where similar and different theoretical methods have been used