Multireference Character for 3d Transition-Metal-Containing Molecules

Coupled cluster and configuration interaction diagnostics have been examined in order to assess the reliability of single reference quantum methods for a series of 3d transition metal species including hydrides, nitrides, chalcogenides, halides, small clusters, coordination complexes, and metal dimers. Several means of diagnostics have been considered including <i>T</i>₁ and <i>D</i>₁ diagnostics (the Frobenius norm and matrix 2-norm of coupled cluster amplitudes for single excitations, respectively), C₀² (the weight of leading configuration of a complete active space wave function), and %TAE (percent total atomization energy). <i>T</i>₁ and <i>D</i>₁ diagnostics are strongly correlated for certain metal–ligand bonding types. The use of <i>T</i>₁ and <i>D</i>₁ together with %TAE can provide more reliable assessment of the severity of nondynamical correlation than a single indicator can provide. New criteria, namely <i>T</i>₁ > 0.05, <i>D</i>₁ > 0.15, and |%TAE| > 10, are suggested to identify inorganic species with substantial nondynamical correlation. For these systems, energies and spectroscopic properties computed using single reference electronic correlation methods may suffer from large errors and unpredictable behavior. Conversely, a computation where a molecule is below one or more of these thresholds does not always imply domination by a single reference. Some historically pathological molecules such as Mn₂ and Cr₂ show <i>T</i>₁ < 0.05 and <i>D</i>₁ < 0.15. Current implementations of coupled cluster diagnostics may still be insufficient for categorization of molecules that have pronounced nondynamical correlation