What Levels of Coupled Cluster Theory Are Appropriate for Transition Metal Systems? a Study Using near Exact Quantum Chemical Values for 3d Transition Metal Binary Compounds.

Transition metal compounds are traditionally considered to be challenging for standard quantum chemistry approximations like coupled cluster (CC) theory, which are usually employed to validate lower level methods like density functional theory (DFT). To explore this issue, we present a database of bond dissociation energies (BDEs) for 74 spin states of 69 diatomic species containing a 3d transition metal atom and a main group element, in the moderately sized def2-SVP basis. The presented BDEs appear to have an (estimated) 3σ error less than 1 kJ/mol relative to the exact solutions to the non-relativistic Born-Oppenheimer Hamiltonian. These benchmark values were used to assess the performance of a wide range of standard CC models, as the results should be beneficial for understanding the limitations of CC models for transition metal systems. We find that interactions between metals and monovalent ligands like hydride and fluoride are well described by CCSDT. Similarly, CCSDTQ appears to be adequate for bonds between metals and nominally divalent ligands like oxide and sulphide. However, interactions with polyvalent ligands like nitride and carbide are more challenging, with even CCSDTQ(P)_Λ yielding errors on the scale of a few kJ/mol. We also find that many perturbative and iterative approximations to higher order terms either yield disappointing results, or actually worsen the performance relative to the baseline low level CC method, indicating that complexity does not always guarantee accuracy.