Comparison of density functionals for differences between the high- (T2g5) and low- (A1g1) spin states of iron(II) compounds. IV. Results for the ferrous complexes [Fe(L)(‘NHS4’)]

Abstract

Previous work testing density functionals for use in calculating high-spin-low-spin energy differences, deltaE(HL), for iron(II) spin-crossover transitions has tended to conclude that only properly reparametrized hybrid functionals can predict deltaE(HL) since it seems to depend critically on a correct description of the electron pairing energy governed by the exchange term. Exceptions to this rule are the previous three papers (I, II, and III in the present series of papers) where it was found that modern generalized gradient approximations (GGAs) and meta-GGAs could do as well as hybrid functionals, if not better, for this type of problem. In the present paper, we extend these previous studies to five more molecules which are too large to treat with high-quality ab initio calculations, namely, the series [Fe(L)('NHS(4)')], where NHS(4)=2.2'-bis(2-mercaptophenylthio)diethylamine dianion, and L=NH(3), N(2)H(4), PMe(3), CO, and NO(+). Since we know of no reliable experimental estimate of deltaE(HL), we content ourselves with a comparison against the experimentally determined ground-state spin symmetry including, in so far as possible, finite-temperature effects. Together with the results of Papers I, II, and III, this paper provides a test of a large number of functionals against the high-spin/low-spin properties of a diverse set of Fe(II) compounds, making it possible to draw some particularly interesting conclusions. Trends among different classes of functionals are discussed and it is pointed out that there is at least one functional, namely, the OLYP generalized gradient approximation, which is able to give a reasonably good description of the delicate spin energetics of Fe(II) coordination compounds without resorting to hybrid functionals which require the relatively more expensive calculation of a Hartree-Fock-type exchange term.

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

“…(1), must be computed. So far, this has been attempted only by Ganzenmüller et al 31 However, unfortunately, their calculations did not fully account for either the electrostatic contribution to the total energy or the effect of the periodic lattice on the molecular structure.…”

“…28,29 In practice, for each compound, the amount of HF exchange can be varied to fit the desired values for the gaps but no "universally good choice" has been identified so far. The dependence of the adiabatic energy gap on the amount of HF and either of LDA (Slater) or GGA (B88 and OPTX) exchange is explained very clearly in the work of Ganzenmüller et al 31 Finally, it is important to remark that, even when a certain hybrid functional is found to return satisfactory energy gaps, it might not be the optimal functional to describe other properties of the molecule.…”

“…In contrast, some more recent GGA functionals, such as the OLYP (combining the Handy and Cohen's OPTX exchange 27 with the LYP correlation), have been claimed to perform rather well. 28,29,31 Among the meta-GGAs, the Van VoorhisScuseria exchange-correlation functional 30 was tested by Ganzenmüller et al, 31 who concluded that it provided quite accurate results for single-point calculations, while it predicted artificially twisted structures.…”

Computational approach to the study of thermal spin crossover phenomena

“…Nowadays, there is not a definitive DFT functional able to compute ?H HL with sufficient precision for different series of Fe(II) complexes and, in fact, this issue is currently debated in the literature. [45][46][47][48][49][50][51][52][53][54][55][56][57] Multiconfigurational wave function based methods, on the other hand, have proved to be capable of giving accurate values for the LS-HS energy difference but they are computationally much more expensive, particularly in the calculation of optimized geometries and vibrational frequencies. 26,28,29,51,52,[58][59][60][61][62] The aim of this work is to determine through calculations the key parameters of the thermal SCO process, i.e., the zeropoint corrected energy difference between the LS and HS states, ?H HL , the entropy change associated to the spin transition, ?S HL , and an estimation of the transition temperature, T 1/2 , for a set of Fe(II) compounds with ligands of different nature.…”

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).…”