The wave function of the ground 6 A 1 state of the complex FeF 6 3- has been calculated in the Heitler-London scheme with configuration interactions. Our basic configurations are constructed from the eigenfunctions of free ions (Fe 3+ , Fe 2+ , Fe - ) and of free atom F. The Hamiltonian matrix elements have been calculated according to the method of “Atoms in Molecules”, where the empirical atomic energies are inserted appropriately into the matrix elements. As the large ionization energy of Fe 2+ ion (30.64 eV) decreases the excitation energies of the charge transfer configurations, the polarization energy associated with electron transfer becomes a relatively important factor. We have checked the ground state wave function by comparing the calculated hyperfine spin densities and the crystal field splitting parameter 10 D q with the experimental data. Satisfactory agreements between the calculated and observed quantities have been obtained. Some qualitative discussions on the quartet states are also given.