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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Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principle-driven methodologies to model complex chemical and materials processes. Over the past few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach, and outlook.

ABSTRACT:A new charge analysis is presented that gives an accurate description of the electrostatic potential from the charge distribution in molecules. This is achieved in three steps: first, the total density is written as a sum of atomic densities; next, from these atomic densities a set of atomic multipoles is defined; finally, these atomic multipoles are reconstructed exactly by distributing charges over all atoms. The method is generally applicable to any method able to provide atomic multipole moments, but in this article we take advantage of the way the electrostatic potential is calculated within the Density Functional Theory framework. We investigated a set of 31 molecules as well as all amino acid residues to test the quality of the method, and found accurate results for the molecular multipole moments directly from the DFT calculations. The deviations from experimental values for the dipole/quadrupole moments are also small. Finally, our Multipole Derived Charges reproduce both the atomic and molecular multipole moments exactly.

Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary S(N)2@C, S(N)2@Si, and S(N)2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the S(N)2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and postbarrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the S(N)2 barrier, but they also show how electronic effects modulate the barrier height.

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