First-principles calculations of the magnetism of Fe$_2$O$_2$H$_2$
  By expanding the wave function in plane waves, we use the pseudopotential method of density functional theory within the generalized gradient approximation to calculate the effective magnetic coupling energies of the S=5/2 spins in the Fe2 dimer, approximated as Fe$_2$O$_2$H$_2$. Setting the Fe-O bond length at the value corresponding to the minimum total energy, we find the difference in antiferromagnetic and ferromagnetic exchange energies as a function of the Fe-O-Fe bond angle $\theta$. The effective interaction is antiferromagnetic for 63$^{\circ}<\theta<105^{\circ}$, and is ferromagnetic otherwise. Full potential augmented plane wave calculations were also performed at $\theta=100,105^{\circ}$, confirming these results, and providing information relevant to the local anisotropy of the spin interactions. 
Sergey Stolbov, Richard A. Klemm, and Talat S. Rahman