2008b). The study revealed that regardless of learn more whether the spin–orbit coupling (SOC) part of the ZFS was estimated with the Pederson–Khanna or the quasi-restricted

KU55933 in vitro orbitals approach, accounting for the spin–spin (SS) interaction always improves the results. The physical necessity of accounting for the SS interaction is shown from its 30% contribution to the axial D parameters. In general, the calculations were found to overestimate systematically the experimental D values by 60%. The authors call attention to the fact that the signs of calculated axial ZFS parameters are unreliable once E/D > 0.2. Calculated D and E/D values were found to be highly sensitive to small structural changes; disconcertingly, the use of optimized geometries was found to lead to a significant deterioration of theoretical predictions relative to experimental XRD geometries. A subsequent study (Zein and Neese 2008) showed that using the coupled-perturbed spin–orbit coupling (CP-SOC) approach (Neese 2007) together with hybrid DFT functionals

this website leads to a slope of the correlation line between experimental and calculated D values that is essentially unity, provided that the direct SS interaction is properly included in the treatment. For the case of the hyperfine coupling to the metal, DFT performance is not entirely satisfactory (Munzarova and Kaupp 1999; Munzarova et al. 2000). Since this property

involves three contributions (Fermi contact, spin–dipolar, and spin–orbit coupling) Bcl-w which feature different physical mechanisms, it is difficult to calculate all of them simultaneously with quantitative accuracy. Ligand HFCs are easier to compute but, again, results are less accurate than for organic radicals, and errors of 30% must be tolerated (Neese 2001b). Kossmann et al. (2007) investigated the performance of modern DFT functionals for the prediction of molecular hyperfine couplings in extended test calculations for a series of small radicals and transition metal complexes. It was shown that for the prediction of metal and ligand HFCs, TPSS is better than BP86, but more importantly, that the hybrid variant TPSSh is significantly superior to TPSS and probably even better than the “de facto standard” B3LYP functional. The double-hybrid B2PLYP functional also affords accurate predictions, particularly for HFCs of metal nuclei, but the existence of outliers suggests that this method may lack stability. The reliable performance of the TPSSh functional has since received additional confirmation in our recent study (Pantazis et al. 2009) aimed at the analysis of hyperfine coupling parameters in tetramanganese models of the OEC.