Interplay of Correlation and Relativistic Effects in Correlated Calculations on Transition-Metal Complexes: The (Cu₂O₂)²⁺ Core Revisited

Owing to the availability of large-scale computing facilities and the development of efficient new algorithms, wave function-based ab initio calculations are becoming more common in bioinorganic chemistry. In principle they offer a systematic route toward high accuracy. However, these calculations are by no means trivial. In this contribution we address some pertinent points through a systematic theoretical study for the equilibrium between the peroxo- and bis-(μ-oxo) isomers of the [{Cu(C2H8N2)}2O2](2+) complex. While this system is often regarded as a prototypical multireference case, we treat it with the single reference local-pair natural orbital coupled cluster method and reiterate that the multireference character in this system is very limited. A set of intermediate structures, for the interconversion between the two isomers, is calculated through a relaxed surface scan thus allowing the calculation of an energetic profile that cleanly connects the bis-(μ-oxo) and side-on peroxo minima on the ground-state potential energy surface. Only at the highest level of theory involving complete basis set extrapolation, triple excitation contributions as well as relativistic and solvent effects, the bis-(μ-oxo) isomer is found to be slightly more stable than the peroxo structure. This is in agreement with the experimental findings. The effects of basis set, triples excitation, relativity, and solvent contribution have all been analyzed in detail. Finally, the ab initio results are compared with density functional calculations using various functionals. It is demonstrated that the largest part of the discrepancies of the results reported in the literature are due to an inconsistent handling of relativistic effects, which are large in both ab initio and density functional theory calculations.