Strong electron correlation effects are one of the major
challenges
in modern quantum chemistry. Polynuclear transition metal clusters
are peculiar examples of systems featuring such forms of electron
correlation. Multireference strategies, often based on but not limited
to the concept of complete active space, are adopted to accurately
account for strong electron correlation and to resolve their complex
electronic structures. However, transition metal clusters already
containing four magnetic centers with multiple unpaired electrons
make conventional active space based strategies prohibitively expensive,
due to their unfavorable scaling with the size of the active space.
In this work, forefront techniques, such as density matrix renormalization
group (DMRG), full configuration interaction quantum Monte Carlo (FCIQMC),
and multiconfiguration pair-density functional theory (MCPDFT), are
employed to overcome the computational limitation of conventional
multireference approaches and to accurately investigate the magnetic
interactions taking place in a [Co(II)3Er(III)(OR)4] (chemical formula [Co(II)3Er(III)(hmp)4(μ2-OAc)2(OH)3(H2O)], hmp = 2-(hydroxymethyl)-pyridine) model cubane water oxidation
catalyst. Complete active spaces with up to 56 electrons in 56 orbitals
have been constructed for the seven energetically lowest different
spin states. Relative energies, local spin, and spin–spin correlation
values are reported and provide crucial insights on the spin interactions
for this model system, pivotal in the rationalization of the catalytic
activity of this system in the water-splitting reaction. A ferromagnetic
ground state is found with a very small, ∼50 cm–1, highest-to-lowest spin gap. Moreover, for the energetically lowest
states,S= 3–6, the three Co(II) sites exhibit
parallel aligned spins, and for the lower states,S= 0–2, two Co(II) sites retain strong parallel spin alignment.