A Few Comments on the Application of Density Functional Theory to the Calculation of the Magnetic Structure of Oligo-Nuclear Transition Metal Clusters.

First principle calculations of the magnetic structure of high nuclearity clusters appears challenging in order to validate fits of magnetic experiments. Density Functional Theory (DFT)-Broken Symmetry approach pair became, in the past few years, the most widely applied computational tool to investigating the chemical-physical properties of complex systems, in particular magnetic molecular compounds. However, the application of the Broken Symmetry formalism requires the knowledge of the energies of 2(N)/2 single Slater determinants, and this task can easily become difficult for large N. Three main approximations are therefore usually done in order to limit the computational efforts: the model dimer approach (MDA), the doped cluster approach (DCA), and the minimum cluster approach (MCA). The whole cluster approach (WCA) will be also applied as reference and in order to check the importance of spin Hamiltonian high order terms. A systematic comparison between these different approaches has been, therefore, performed. Since this study is aimed for being of help in choosing the best method of calculation, we check here the validity of the above approaches by computing the magnetic structure of some test systems: the tetrahedral system (HeH)4 and linear [Cu(II)]3 and [Mn(II)]4 complexes.