  The EPR parameters g factors and the superhyperfine parameters for the
tetrahedral [FeX_4]^- clusters in AgX (X=Cl, Br) are theoretically investigated
from the perturbation formulas of these parameters for a 3d^5 ion under
tetrahedra by considering both the crystal-field and charge transfer
contributions. The related model parameters are quantitatively determined from
the cluster approach in a uniform way. The g-shift \Delta g (=g-g_s, where
g_s\approx 2.0023 is the spin only value) from the charge transfer contribution
is opposite (positive) in sign and much larger in magnitude as compared with
that from the crystal-field one. The importance of the charge transfer
contribution increases rapidly with increasing the covalency and the spin-orbit
coupling coefficient of the ligand and thus exhibits the order of AgCl<AgBr.
The unpaired spin densities of the halogen ns, np\sigma\ and np\pi\ orbitals
are quantitatively determined from the related molecular orbital coefficients
based on the cluster approach.
