Weakly coupled transition-metal centres: high-level calculations on a model Fe(IV)-Fe(IV) system

We briefly review different approaches to calculating the Heisenberg exchange coupling between two weakly coupled metal centres in a complex, with particular emphasis on using multiconfigurational methods to treat individually all the states that arise, without the use of symmetry breaking to obtain a mixed state in which the spins are coupled antiferromagnetically. We illustrate the multiconfigurational approach first using very accurate calculations on two weakly interacting high-spin nitrogen atoms, and then we use these very accurate results to calibrate broken-symmetry single configuration approaches, including density-functional theory with various different functionals. We then consider a dinuclear iron complex chosen to reproduce qualitatively the environment around the diiron-dioxygen "diamond core" in soluble methane monooxygenase hydroxylase. Agreement between high-level multiconfigurational methods and broken-symmetry density-functional theory is only fair.