Cluster models of increasing complexity have been used to model magnetic interactions, in KNiF3. These dusters contain two or four magnetic centers plus the bridge F- anions and different representations of the remaining of the crystal. The magnetic coupling constant has been obtained by computing nb initio wave functions for different spin states. These wave functions explicitly include internal and external correlation effects : Several sets of Gaussian functions have been tested and many sets of molecular orbitals have been considered in order to study the physical origin of magnetism in KNiF3. The calculated magnetic coupling constant differs from model to model but shows a fairly good convergence to the experimental result. The use of different cluster models permits to separate the magnetic coupling constant in several contributions. These are the delocalization of magnetic orbitals, the external correlation, and the collective effects normally hidden in the two body operator of the Heisenberg Hamiltonian.