Spin-state energetics of manganese spin crossover complexes: Comparison of single-reference and multi-reference ab initio approaches

Abstract

Manganese spin crossover (SCO) complexes form a small but ever expanding family of compounds with thermally accessible states of different electronic configuration and total spin. Accurate prediction of spin-state energetics is essential for the theoretical description of these systems. However, this represents a challenging problem that necessitates recourse to correlated wave function methods rather than the more approximate density functional theory (DFT). Here we present a detailed study of spin-state energetics for eight Mn(III) and Mn(II) SCO complexes using the domain-based local pair natural orbital approach to coupled cluster theory with singles, doubles, and perturbative triples, DLPNO-CCSD(T). The effects of reference determinants, basis set, triples excitations, and pair natural orbitals (PNO) thresholds are evaluated and analysed in detail, enabling us to propose a robust and efficient computational protocol based on a combined and balanced mix of extrapolation to the complete basis set and infinite PNO space limits. The results are subsequently used to evaluate multireference wavefunction-based (CASSCF/NEVPT2) and DFT approaches, highlighting their inability to provide a balanced description of spin-state energetics for these complexes. The DLPNO-CCSD(T) protocol proposed in this study can serve as a generally applicable reference-quality quantum chemical method for studying spin crossover systems.