The performance of nonhybrid density functionals for calculating the structures and spin states of Fe(II) and Fe(III) complexes

Abstract

The local density approximation and a range of nonhybrid gradient corrected density functionals (PW91, BLYP, PBE, revPBE, RPBE) have been assessed with respect to the prediction of geometries and spin-state energy preferences for a range of homoleptic Fe(II)L6 and Fe(III)L6 complexes, where L = Cl-, CN-, NH3, pyridine, imidazole, H2O, O=CH2 and tetrahydrofuran. While the qualitative spin-state energies from in vacuo structure optimizations are reasonable the geometries are relatively poorly treated, especially for [FeCl6]3-/4-. Structural results for all the complexes are significantly improved by including environmental effects. The best compromise between structural and spin-state predictive accuracy was obtained for the RPBE functional in combination with the COSMO solvation approach. This approach systematically overestimates the energetic preference for a low spin state, which is partly due to the well-known effect of the lack of exact exchange in nonhybrid functionals and partly due to the larger solvation stabilization of low-spin complexes that have shorter bond lengths and thus smaller molecular volumes than their high-spin partners. Calculations on low spin [Fe(bipy)3]2+ and [Fe(phen)3]2+ and their ortho methyl substituted analogs, which are high spin at room temperature but cross over to low spin at low temperature, suggest the RPBE/COSMO combination generates low spin states which are too stable by approximately 13 kcal mol(-1).

High‐spin versus broken symmetry—Effect of DFT spin density representation on the geometries of three diiron (III) model compounds

“…How to approach this problem within single-configurational DFT is still an area of active inquiry. 49,[53][54][55][56][57] Spin Density Representation and Geometry…”

A Theoretical Study of the 3d‐M(smif)2Complexes: Structure, Magnetism, and Oxidation States

“…Consequently, they systematically favor low-spin states. [31] The popular hybrid functionals benefit from a cancellation of these effects by incorporating some HF exchange. However, by adjusting its amount, the computed spin state ordering and gaps can be arbitrarily tuned, [32,33] which is an obvious deficiency.…”

Spin‐State Energetics of FeII Complexes – The Continuing Voyage Through the Density Functional Minefield

“…[29,30,34,35] Fortunately, this may not be a serious shortcoming, because the trends are often correct, and thus, as described by Vargas et al, [34] a more accurate absolute picture can be obtained by "anchoring" the calculations with an example where the spin-state energetics are accurately known. [34] We [36] . However, varying the whole ligand set alters the energies a lot and is thus not a very exacting test.…”