Toward Quantitative Computations of Exchange Coupling Constants in Transition-Metal Complexes via Multireference Driven Similarity Renormalization Group

The accurate prediction of magnetic exchange coupling constants (<i>J</i>) in transition-metal complexes remains challenging for the electronic-structure theory. In this work, we demonstrate that the state-average driven similarity renormalization group second-order perturbation theory (SA-DSRG-PT2), combined with the density matrix renormalization group, provides quantitative <i>J</i> values on a set of 21 3d bimetallic complexes with a focus on dicopper species. Active orbitals are systematically identified using the atomic valence active space approach, ensuring a consistent inclusion of metal 3d and 4d shells and the ligand orbitals involved in superexchange pathways. Analysis of the SA-DSRG-PT2 effective Hamiltonians indicates that the reduction of the on-site Coulomb repulsion is the primary origin of the improved exchange couplings. In addition, we adopt the orbital mutual correlation plot as an intuitive tool to visualize exchange pathways, allowing direct identifications of metal-metal and metal-ligand correlations and their modulation by dynamical correlation. These results establish SA-DSRG-PT2 as a robust and practical multireference approach for studying magnetic interactions in transition-metal complexes.