Argonne National Laboratory

Research Highlight | Materials Science

Getting the right spin on Fe-complexes
In a study published in Journal of Chemical Theory and Computation, researchers indicate density-corrected DFT is both significantly more accurate and reliable and yields a consistent prediction for the Fe–Porphyrin complex.
Isosurface plots of difference densities at ±0.1 Å–3 for the LS [Fe(CO)6]2+ complex with gold (blue) +0.1 (−0.1) Å–3: (left) nHF – nPBE; (middle) nDMC – nPBE; (right) nDMC – nHF.

Scientific Achievement

Standard quantum chemical methods (CCSD(T)) often yield good agreement with standard DFT (B3LYP) calculations for spin energy differences, but both can be wrong.  Quantum Monte Carlo (QMC) simulations show this for several small Fe complexes.

Significance and Impact

QMC also showed that a simple fix, density-corrected density functional theory, works and can then be used in much larger applications, such as spintronics.

Research Details

  • We calculated energy differences between high-spin and low-spin states of [Fe(X)6]2+, where X is NCH, NH3, H2O, and CO
  • We demonstrated that results from diffusion Monte Carlo methods were well converged; results from quantum chemical [CCSD(T)] and standard density functional methods gave poor results in comparison.
  • Density-corrected density functional calculations using the Hartree-Fock density to calculate energies within density functional theory reduced the error in energy difference by up to a factor of three.

DOI: 10.1021/acs.jctc.7b01196

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