Towards reliable and efficient modeling of [Cu₂O₂]²⁺-based compound electronic structures with the partially fixed...

This work reports a computationally efficient approach for reliable modeling of complex electronic structures based on [Cu₂O₂]²⁺ moieties. Specifically, we explore the recently developed partially fixed reference space (PFRS) protocol to minimize the active space size, taking into account the double d-shell effects. We show that the ground-state electronic structure of the core [Cu₂O₂]²⁺ model system is dominated by the d⁹/d¹⁰ occupations. The PFRS-crafted active spaces are further used to generate the reference wave functions for the multi-reference coupled cluster, configuration interaction, and multi-reference perturbation theory calculations. Specifically, we demonstrate that the bare [Cu₂O₂]²⁺ core can be modeled qualitatively using active spaces as small as CAS(2,2)PFRS. To obtain quantitative agreement with the reference DMRG(32,62)CI calculations, the CAS(4,4) has to be used in conjunction with the MRCCSD correction on top of it. This reliable and computationally efficient protocol is further used to model the electronic structure and properties of ammonia coordinated [Cu₂O₂]²⁺ complexes. Finally, based on the large amount of available experimental data regarding the oxo-peroxo equilibrium of [Cu₂O₂]²⁺-based systems, it is possible to formulate educated guesses regarding the effect of each experimental variable over each d-occupancy-specific state. With a large sample size of d-occupancy-specific state dependence with ligands and solvents, it should be possible to propose new ligands with specific d-occupancy and, therefore, oxidative properties based on the d-occupancy energy gaps of relatively low-cost calculations.