A Review of Density Functional Models for the Description of Fe(II) Spin-Crossover Complexes

Abstract

Spin-crossover (SCO) materials have for more than 30 years stood out for their vast application potential in memory, sensing and display devices. To reach magnetic multistability conditions, the high-spin (HS) and low-spin (LS) states have to be carefully balanced by ligand field stabilization and spin-pairing energies. Both effects could be effectively modelled by electronic structure theory, if the description would be accurate enough to describe these concurrent influences to within a few kJ/mol. Such a milestone would allow for the in silico-driven development of SCO complexes. However, so far, the ab initio simulation of such systems has been dominated by general gradient approximation density functional calculations. The latter can only provide the right answer for the wrong reasons, given that the LS states are grossly over-stabilized. In this contribution, we explore different venues for the parameterization of hybrid functionals. A fitting set is provided on the basis of explicitly correlated coupled cluster calculations, with single- and multi-dimensional fitting approaches being tested to selected classes of hybrid functionals (hybrid, range-separated, and local hybrid). Promising agreement to benchmark data is found for a rescaled PBE0 hybrid functional and a local version thereof, with a discussion of different atomic exchange factors.

Ab Initio Methods in First‐Row Transition Metal Chemistry

“…[148] We also did not summarize studies focusing on three famous SCO models ([Fe(H 2 O) 6 ] 2 + , [Fe(NH 3 ) 6 ] 2 + , and [Fe(NCH) 6 ] 2 + ) as their experimental (negative) quintet-singlet splittings are unknown. These three systems have been extensively tested with various ab initio methods such as CCSD(T), [9,149,150] DLPNO-CCSD(T), [151] spectroscopy oriented CI (SORCI), [152] CASPT2, [95,153] NEVPT2, [44,154] MRCISD + Q, [37] diffusion Monte Carlo, [155][156][157] and DSRG-MRPT. [133] As most of SCO complexes listed in Table 2 exhibit mild multireference character, CCSD(T) is expected to give superior results to multireference methods.…”

Spin-State Splittings in 3d Transition-Metal Complexes Revisited: Benchmarking Approximate Methods for Adiabatic Spin-State Energy Differences in Fe(II) Complexes

“…20−23 However, the results of these studies differ much more from each other than should be the case for the desired benchmarkquality method. 24 Large deviations between very recent DMC results and CCSD(T) of up to 70 kJ/mol 23 have raised further doubts, 12,24 especially since it can be argued that the static correlation in the Fe(II) complexes studied in that work should actually be covered reasonably well at the CCSD(T) level (see also below). 25−27 Although DMC has the potential to give very accurate total energies, such calculations are based on the fixed-node approximation and therefore rely crucially on the quality of the trial wave function used to generate the nodes of the final wave function.…”

Evolution of Vibrational Spectra in the Manganese–Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13]+

“…The influence of Hartree–Fock exchange (present in hybrid functionals such as the popular B3LYP) on the computed energies of different spin states is well documented: larger proportions of Hartree–Fock exchange tend to favor unpaired spin density and hence higher multiplicities. 4 Less well documented, but of particular relevance here, is the fact that even when the multiplicity is fixed, functional choice can have a substantial impact on the relative energies of different local minima, particularly in cases where the nature of the chemical bonding differs qualitatively between them. In the family of M@Si 14 clusters (where M is the encapsulated metal dopant), for example, the BLYP functional (and its hybrid, B3LYP) tends to favor “open” structures with three-connected vertices while the PBE functional (and its hybrid, PBE0), in contrast, stabilizes deltahedral structures with more highly connected vertices.…”

“…There is, however, still no clear consensus on what the “best” functional is for clusters of this type, and the choice is often motivated by the authors’ previous success in related studies. The influence of Hartree–Fock exchange (present in hybrid functionals such as the popular B3LYP) on the computed energies of different spin states is well documented: larger proportions of Hartree–Fock exchange tend to favor unpaired spin density and hence higher multiplicities . Less well documented, but of particular relevance here, is the fact that even when the multiplicity is fixed, functional choice can have a substantial impact on the relative energies of different local minima, particularly in cases where the nature of the chemical bonding differs qualitatively between them.…”