Simulation of Heme using DFT+U: a step toward accurate spin-state energetics

TL;DR

This paper demonstrates that the DFT+U method with a U parameter around 4 eV accurately predicts spin states and geometries of iron heme complexes, outperforming standard DFT approaches and offering a cost-effective alternative.

Contribution

It introduces the application of DFT+U to iron heme complexes, showing improved accuracy in spin-state energetics and geometries over traditional DFT methods.

Findings

DFT+U with U ≈ 4 eV matches experimental spin states.

Standard DFT fails to accurately describe iron heme complexes.

Self-consistent U calculation tends to overestimate the optimal U value.

Abstract

We investigate the DFT+U approach as a viable solution to describe the low-lying states of ligated and unligated iron heme complexes. Besides their central role in organometallic chemistry, these compounds represent a paradigmatic case where LDA, GGA, and common hybrid functionals fail to reproduce the experimental magnetic splittings. In particular, the imidazole pentacoordinated heme is incorrectly described as a triplet by all usual DFT flavors. In this study we show that a U parameter close to 4 eV leads to spin transitions and molecular geometries in quantitative agreement with experiments, and that DFT+U represents an appealing tool in the description of iron porphyrin complexes, at a much reduced cost compared to correlated quantum-chemistry methods. The possibility of obtaining the U parameter from first-principles is explored through a self-consistent linear-response formulation. We find that this approach, which proved to be successful in other iron systems, produces in this case some overestimation with respect to the optimal values of U.