  In this work we present a model for the determination of the interaction
energy for triplet and singlet states in atoms with incomplete filled shells.
Our model includes the modification of the Coulomb's law by the interaction
between the magnetic moments of the electrons and a Heisenberg term. We find
that the energy of the triplet state is lower than the energy of the singlet
state. We calculate the interaction energy between the electrons from the
adjacent atoms in fcc and bcc lattices and we find that the minimum interaction
energy is attained for the triplet state. The result is presented for the
interaction between the electrons of the first coordination group. The
interaction energy which aligns the spins is used to evaluate the Curie
temperature in a mean field model. Compared to previous models, our simple
model fits the experimental data.
