Magnetostructural dynamics with the extended broken symmetry formalism: Antiferromagnetic [2Fe-2S] complexes

A general spin-projection framework is laid out which allows one to perform ab initio molecular dynamics simulations of antiferromagnetically coupled spin dimers. The method extends the well-established broken-symmetry formalism and is systematically and consistently improvable. It allows for accessing structure within the same spin-projection approximation as employed to compute the exchange coupling constant J of such complexes in their low-spin state as a function of time. The resulting time evolution of the exchange coupling, J(t), can be analyzed most conveniently in terms of the corresponding power spectrum, J(omega), thus giving access to dynamical magnetostructural properties. The method has been implemented using a well-tested approximation to spin-projection and was applied to a minimal [2Fe-2S] model, i.e. to the [Fe2S2(SH)(4)](2-) complex at 300 K in vacuo. Thermal fluctuations at room temperature are found to change the antiferromagnetic coupling J by about 50% with respect to the average value, and the features of its power spectrum, J(omega), can be traced back to a coupling of J to particular vibrational modes.