Multi-configurational approaches yield universal wave functionparameterizations that can qualitatively well describe electronic structuresalong reaction pathways. For quantitative results, multi-reference perturbationtheory is required to capture dynamic electron correlation from the otherwiseneglected virtual orbitals. Still, the overall accuracy suffers from the finitesize and choice of the active orbital space and peculiarities of theperturbation theory. Fortunately, the electronic wave functions at equilibriumstructures of reactants and products can often be well described bysingle-reference methods and hence are accessible to accurate coupled clustercalculations. Here, we calculate the heterolytic double dissociation energy offour 3d-metallocenes with the complete active space self-consistent fieldmethod and compare to highly accurate coupled cluster data. Our coupled clusterdata are well within the experimental error bars. This accuracy can also beapproached by complete active space calculations with an orbital selectionbased on information entropy measures. The entropy based active space selectionis discussed in detail. We find a very subtle balance between static anddynamic electron correlation effects that emphasizes the need for algorithmicactive space selection and that differs significantly from restricted activespace results for identical active spaces reported in the literature.
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