Benchmarks and reliable DFT results for spin-crossover complexes

TL;DR

This paper evaluates the accuracy of various computational methods for spin-crossover complexes, demonstrating that density-corrected DFT with Hartree-Fock densities offers reliable results and benchmarks for these challenging systems.

Contribution

It introduces the use of all-electron fixed-node diffusion Monte Carlo and density-corrected DFT with Hartree-Fock densities as accurate benchmarks for spin-crossover complexes.

Findings

All-electron diffusion Monte Carlo provides converged benchmarks.

HF-DFT significantly improves accuracy over standard DFT.

HF-DFT eliminates the parameter-dilemma in spin-crossover calculations.

Abstract

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.