Towards understanding performance differences between approximate density functionals for spin states of iron complexes

Abstract

Density functional theory has been widely used to investigate the structural and electronic properties of heme-containing proteins such as cytochrome P450. Nevertheless, recent studies have shown that approximate exchange-correlation energy density functionals can incorrectly predict the stability order of spin states in, for instance, iron-containing pyridine and imidazole systems. This raises questions about the validity of earlier theoretical studies. In this work, we systematically investigate a few typical inorganic and organic iron-containing complexes and try to understand the performance difference of various density functionals. Two oxidation states of iron, Fe(II) and Fe(III), with different spin states and both adiabatic and vertical structures are considered. A different description of the outmost molecular orbital is found to play the crucial role. Local density and generalized gradient based functionals bias the lower spin state and produce a more localized frontier orbital that is higher in energy than the hybrid functionals. Energy component analysis has been performed, together with comparison of numerous structural and electronic properties. Implications of the present work to the theoretical study of heme-containing biological molecules and other spin-related systems are discussed.

Iron(II) and Nickel(II) Complexes of N‐Alkylimidazoles and 1‐Methyl‐1H‐1, 2, 4‐Triazole: X‐ray Studies, Magnetic Characterization, and DFT Calculations

“…10Dq(Fe) ≈ 10Dq(Ni)×1.0526. The values in brackets refer to the 3 A 2g → 3 T 2g transition of the corresponding nickel complexes.…”

Structural and Dynamic Properties of (SiO2)6 Silica Nanostructures: A Quantum Molecular Dynamics Study

“…In DFT, 13 the total energy difference between two structures, ∆E, can be expressed as [26][27][28] where 28 and where E, T, V ne , V nn , J, E x , and E c stand for the total energy, total kinetic energy, nuclear-electron attraction, nuclear-nuclear repulsion, classical Coulomb repulsion among electrons, exchange, and correlation energy, respectively. The reference structure is the global minimum from the present study.…”

“…The energy component analysis has been used elsewhere [26][27][28] as a powerful tool to understand the physical nature of spinrelated phenomena and will be employed in this study to determine if there is any dominant contribution from one or few energy components to dictate to the total energy difference among a series of local minima. In DFT, 13 the total energy difference between two structures, ∆E, can be expressed as [26][27][28] where 28 and where E, T, V ne , V nn , J, E x , and E c stand for the total energy, total kinetic energy, nuclear-electron attraction, nuclear-nuclear repulsion, classical Coulomb repulsion among electrons, exchange, and correlation energy, respectively.…”

Exploring the origin of the internal rotational barrier for molecules with one rotatable dihedral angle

“…[55][56][57][58][59] In the adiabatic dihedral angle rotation, both staggered and eclipsed conformers are in their respective optimized structure, whereas in the vertical case, bond lengths and angles for the two conformers are fixed to be identical except for the changing dihedral angle. For the adiabatic rotation, each time the dihedral angle of the two conformers is altered, a geometrical optimization with that dihedral angle will be performed.…”