Relative energy of the high-(T2g5) and low-(A1g1) spin states of the ferrous complexes [Fe(L)(NHS4)]: CASPT2 versus density functional theory

Abstract

High-level ab initio calculations using multiconfigurational perturbation theory [complete active space with second-order perturbation theory (CASPT2)] were performed on the transition energy between the lowest high-spin (corresponding to (5T2g) in Oh) and low-spin (corresponding to 1A1g in Oh) states in the series of six-coordinated Fe(II) molecules [Fe(L)(NHS4)], where NHS4 is 2,2'-bis(2-mercaptophenylthio)diethylamine dianion and L=NH3, N2H4, PMe3, CO, and NO+. The results are compared to (previous and presently obtained) results from density functional theory (DFT) calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 (2004); Ganzenmuller et al., ibid. 122, 234321 (2005); Fouqueau et al., ibid. 122, 044110 (2005); Lawson Daku et al., ChemPhysChem 6, 1393 (2005)] to perform well for the spin-pairing problem in these and other Fe(II) complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 (2006)]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet (free) energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.

Assessment of density functional theory for iron(II) molecules across the spin-crossover transition

“…For example, the OLYP functional, whose exchange part (OPTX) is parametrized to reproduce the HartreeFock exchange for atoms, 27 PBE0. 49 Unfortunately, however, OLYP is not as accurate as the hybrids for predicting geometry optimizations and bondlengths. 49 Another such GGA functionals, which was found to perform at the level of the hybrids (if not even better), 29 is the HCTH407.…”

“…49 Unfortunately, however, OLYP is not as accurate as the hybrids for predicting geometry optimizations and bondlengths. 49 Another such GGA functionals, which was found to perform at the level of the hybrids (if not even better), 29 is the HCTH407. 70 Our own results are then listed in Table XIV 2+ can be described at all by the single-determinant picture provided by DFT.…”

“…In some interesting works, 14,28,29,48,49 wave-function based methods were considered (see discussion below). However, unfortunately, the authors themselves admitted that their results were plagued by a number of systematic errors.…”

Influence of Guest−Host Interactions on the Structural, Energetic, and Mössbauer Spectroscopy Properties of Iron(II)tris(2,2′-bipyridine) in the Low-Spin and High-Spin States: A Density-Functional Theory Study of the Zeolite-Y Embedded Complex

“…However, although they perform quite well for the study of numerous properties of transition metal complexes, 59 they tend to dramatically fail when it comes to the accurate evaluation of the relative energies of the different spin-states of these systems. 6,7,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76] [L], respectively, where r Γ designates the average iron-nitrogen bond length in the Γ manifold (Γ ) LS, HS).…”

The Role of Self-Interaction Corrections, Vibrations, and Spin-Orbit in Determining the Ground Spin State in a Simple Heme

“…For example, Fe-and Cu-porphyrin-like macrocycles carry oxygen for mammals and crustaceans, respectively, and Photosystem II, the oxygen evolving complex, is composed of spin-carrying Mn centers. For natural molecular systems, the chemical functions are impacted by varying spin states and/or charge states of the transition-metal system, and this is particularly important for the mammalian oxygen-transporting mechanisms that are accomplished by Heme-like molecules [1][2][3][4][5][6][7][8]. A seemingly important characteristic of these molecules is that the multiple spin-and charge-states have similar degrees of stability/instability at room temperature leading to flexible transitions between different states.…”