Sum-over-states based multireferenceab initio calculation of EPR spin Hamiltonian parameters for transition metal complexes. A case study

The recently developed spectroscopy oriented multireference configuration interaction variant (SORCI) is applied to the problem of the prediction of electron paramagnetic resonance parameters in transition metal complexes within a sum-over-states (SOS) framework. The prototypical complex Cu(NH(3))(4)(2+) is taken as an example in order to test the validity and convergence properties of several computational approximations. The results show that the SORCI method affords accurate results for the g-tensor, the metal dipolar hyperfine coupling (HFC), the spin-orbit coupling contribution to the metal HFC and the isotropic and dipolar ligand HFCs. Accurate prediction of the isotropic metal HFC is challenging and requires flexible basis sets in the core region together with explicit inclusion of the core electrons and high-lying core-like virtual orbitals in the CI. Comparison of SORCI and density functional theory (DFT) results reveals that the latter are far off experiment (g-tensor) or achieve reasonable results only through a fortunate cancellation of large errors (metal HFC). The ligand HFC is overestimated by DFT owing to overly covalent bonding which leads to too much spin transfer on to the ligands.