Benchmarks and Reliable DFT Results for Spin Gaps of Small Ligand Fe(II) Complexes

All-electron fixed-node diffusion Monte Carlo provides benchmark spin gaps for four Fe­(II) octahedral complexes. Standard quantum chemical methods (semilocal DFT and CCSD­(T)) fail badly for the energy difference between their high- and low-spin states. Density-corrected DFT is both significantly more accurate and reliable and yields a consistent prediction for the Fe–Porphyrin complex.