Efficient state-interaction approach for the -matrix analysis in high-spin molecules.

We present an efficient state-interaction approach for evaluating -shifts in high-spin molecular systems. Using a spin-orbit-coupled effective Hamiltonian with a restricted active space configuration interaction wavefunction, this method captures key excited-state contributions to -shifts without requiring large orbital spaces, maintaining computational efficiency. Additionally, we introduce a property-driven algorithm to automatically select relevant orbitals, optimizing the active space selection. Application to diatomic and conjugated organic molecules demonstrates accuracy comparable to advanced methods, providing detailed insight into the origins of -shifts. This methodology offers a flexible, efficient tool for exploring magnetic properties in complex molecules.