The structural, electronic and magnetic properties of the perovskite s
ystems KMF3 (M = Mn, Fe, Co, Ni) have been investigated with CRYSTAL95
, a periodic ab initio Hartree-Fock program. An all-electron Gaussian
basis set has been used. The equation of state has been determined fir
st for the cubic structure; then deviations from cubic symmetry have b
een explored, with the result that the Mn, Fe and Co systems are found
to be slightly more stable in a tetragonal geometry. The systems are
almost fully ionic, with net charges for K and M of + 1, ca. -0.9 and
cn. + 1.8 \e\, respectively. The antiferromagnetic (AFM) is always mor
e stable than the ferromagnetic (FM) phase; the energy difference Delt
a E = E(FM) - E(AFM) is shown: (a) to be additive with respect to the
number of M-M first neighbours; (b) to increase with decreasing lattic
e parameter according to an inverse power law; and (c) to become zero
when the M-(F) over cap-M angle approaches 90 degrees. The super-excha
nge coupling constants, evaluated from AE by using an Ising model hami
ltonian, are in qualitative agreement with the experimental data (from
30% to 45% of the latter). Mulliken population data, charge and spin
density maps and density of states are used to illustrate the electron
ic structure.