On the impact of multi-reference character of small transition metal compounds on their bond dissociation energies.

Determining the multi-reference character of a molecular system and its impact on the limits within which its properties may be calculated accurately by different quantum chemical methods remains a difficult yet important task in computational chemistry. Especially, transition metal compounds continue to frequently provide a challenge to quantum chemists in this regard. In this work, we construct, analyze, and evaluate different computational protocols to determine the impact of the multi-reference character of transition metal compounds on their bond dissociation energies using a set of reference data for 60 diatomic molecules. We find that the fractional orbital density approach allows to determine two global indicators on a physically sound basis. These can subsequently be used to classify the assessed set of molecules with high accuracy into categories of systems for which their multi-reference character matters substantially for their bond dissociation energies and for which it does not. A comparison with earlier suggested thresholds for classification of molecular systems due to their multi-reference character suggests that our approaches yield substantially better performance.