State-interaction approach for evaluating g-tensors within EOM-CC and RAS-CI frameworks: Theory and benchmarks
Among various techniques designed for studying open-shell species, electron paramagnetic resonance (EPR) spectroscopy plays an important role. The key quantity measured by EPR is the g-tensor describing the coupling between external magnetic field and molecular electronic spin. Rigorous theoretical framework for quantum chemistry calculations of g-tensors is based on response theory, which involves substantial developments that are specific to underlying electronic structure models. A simplified and easier to implement approach is based on the state-interaction scheme. We describe and benchmark a (state-interaction) quasi-degenerate perturbation theory approach using equation-of-motion coupled-cluster and restricted-active-space configuration interaction wave functions. The analysis confirms that this approach can deliver accurate results and highlights caveats of applying it, such as a choice of the reference state, convergence with respect to the number of states used in calculations, etc. The analysis also contributes towards better understanding of challenges in calculations of higher-order properties using approximate wave functions.