  DFT is used throughout nanoscience, especially when modeling spin-dependent
properties that are important in spintronics. But standard quantum chemical
methods (both CCSD(T) and self-consistent semilocal density functional
calculations) fail badly for the spin adiabatic energy difference in Fe(II)
spin-crossover complexes. We show that all-electron fixed-node diffusion Monte
Carlo can be converged at significant computational cost, and that the B3LYP
single-determinant has sufficiently accurate nodes, providing benchmarks for
these systems. We also find that density-corrected DFT, using Hartree-Fock
densities (HF-DFT), greatly improves accuracy and reduces dependence on
approximations for these calculations. The small gap in the self-consistent DFT
calculations for the high-spin state is consistent with this. For the spin
adiabatic energy differences in these complexes, HF-DFT is both accurate and
reliable, and we make a strong prediction for the Fe-Porphyrin complex. The
"parameter-dilemma" of needing different amounts of mixing for different
properties is eliminated by HF-DFT.
