Thole's modified dipole interaction model for constructing molecular polarizabilities from effective, isotropic atomic polarizabilities is reviewed and extended. We report effective atomic polarizabilities for H, C, N, O, S, and the halogen atoms, independent of their chemical environment. They are obtained by fitting the model both to experimental and calculated molecular polarizabilities, the latter to enable one to model ab initio polarizabilities for various basis sets.

A critical assessment of the OPBE functional is made for its performance for the geometries and spin-states of iron complexes. In particular, we have examined its performance for the geometry of first-row transition-metal (di)halides (MnX2, FeX2, CoX2, NiX2, CuX, X=[F, Cl]), whose results were previously [J. Chem. Theory Comput. 2006, 2, 1282] found to be representative for a much larger and more diverse set of 32 metal complexes. For investigating the performance for spin ground-states of iron complexes, we examined a number of small iron complexes (Fe(II)Cl4(2-), Fe(III)Cl4(1-), Fe(II)Cl6(4-), Fe(III)Cl6(3-), Fe(II)CN6(4-), Fe(III)CN6(3-), Fe(VI)O4(2-), Fe(III)(NH3)6(3+)), benchmark systems (Fe(II)(H2O)6(2+), Fe(II)(NH3)6(2+), Fe(II)(bpy)3(2+)), and several challenging iron complexes such as the Fe(II)(phen)2(NCS)2 spin-crossover compound, the monopyridylmethylamine Fe(II)(amp)2Cl2 and dipyridylmethylamine Fe(II)(dpa)2(2+), and the bis complex of Fe(III)-1,4,7-triazacyclononane (Fe(III)((9)aneN3)2(3+). In all these cases OPBE gives excellent results.

Spin state energies of iron complexes are important for biochemical applications such as the catalytic cycle of cytochrome P450. Due to the size of these systems and the presence of iron, accurate computational results can be obtained only with density functional theory (DFT). Validation of exchange−correlation (xc) DFT functionals for predicting the correct spin ground state of iron complexes is a rather unexplored area. In this contribution we report a systematic study on the performance of several xc functionals for seven iron complexes that are experimentally found to have either a low, intermediate, or high spin ground state. Standard xc functionals like LDA, BLYP, and PBE are found to disfavor high spin states, whereas hybrid and some meta-GGA functionals do provide the correct spin ground state for all molecules. Recently improved pure DFT functionals such as Handy's optimized exchange (OPTX) also perform well. The origin for the apparent performance of the DFT functionals has been addressed and seems to be related to the inclusion of fourth-order terms (s 4) of the dimensionless (or reduced) density gradient s in the exchange functional.

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