  Based on density functional calculations, we report on the orbital order and
microscopic magnetic model of FeNCN, a prototype compound for orbital-only
models. Despite having a similar energy scale, the spin and orbital degrees of
freedom in FeNCN are only weakly coupled. The ground-state configuration
features the doubly occupied d_{3z^2-r^2} (a1g) orbital and four
singly-occupied d orbitals resulting in the spin S=2 on the Fe+2 atoms, whereas
alternative (Eg') configurations are about 75 meV/f.u. higher in energy.
Calculated exchange couplings and band gap are in good agreement with the
available experimental data. Experimental effects arising from possible orbital
excitations are discussed.
