Active Space Methods In Electronic Structure theory and Applications To Gas Separations In Metal-Organic Frameworks

Abstract

Active space methods such as complete active space self-consistent field theory (CASSCF) are applied to many systems of interest with a focus on the challenges in choosing orbitals for active spaces. The systematic exploration of active spaces is considered from the standpoint of theoretical development, specifically the benchmarking of generalized active space self-consistent field theory and SplitGAS on a variety of systems. Additionally, a “correlated participating orbital” active space selection scheme is applied to CASSCF and restricted active space self-consistent field theory followed by second-order perturbation theory (CASPT2 and RASPT2, respectively) for singlet-triplet splittings of diradical organic molecules. “πCPO” is introduced as an effective and economical option for π-system excitation energies. It is also demonstrated that multiconfiguration pair-density functional theory (MC-PDFT) can provide good agreement with CASPT2 at a much lower computational cost. The computational affordability of MC-PDFT is also shown through the calculation of the full spin ladder of Fe2S2 compounds for which second-order perturbation theory could only be performed for high-spin states. The effects of including high local exchange (HLE) modifications to the MC-PDFT exchange and correlation energies for the relative spin-state energies of several other iron complexes is examined. The remainder of the work features gas separations in metal-organic frameworks (MOFs), beginning with CO2 capture in a copper paddle-wheel MOF and continuing to metal-catecholates, which are studied using Kohn-Sham density functional theory and CASPT2 in comparisons of different first-row transition metals for the capture of toxic NO and for O2/N2 separation. Finally, a screening study identifies specific MOF structures for metal-catecholate modification as synthetic targets for the purpose of O2/N2 separation.