Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. The interaction between the reactants and products electronic states near the crossing-point configuration is 0.20 eV and is consistent with an adiabatic electron-transfer mechanism. Electron transfer is predicted to possess a small positive activation energy (0.11 eV) that is in excellent agreement with values deduced from conductivity measurements. Measured electron mobility can be explained in terms of nearest-neighbor electron hops without significant contribution from iron atoms further away. Comparison of the predicted maximum polaron binding energy with the predicted half bandwidth indicates compliance with the small-polaron condition. Therefore the localized electron treatment is appropriate to describe electron transport in this system.

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.

Methyl-coenzyme M reductase, the rate-limiting enzyme in methanogenesis and anaerobic methane oxidation, is responsible for the biological production of more than 1 billion tons of methane per year. The mechanism of methane synthesis is thought to involve either methyl-nickel(III) or methyl radical/Ni(II)-thiolate intermediates. We employed transient kinetic, spectroscopic, and computational approaches to study the reaction between the active Ni(I) enzyme and substrates. Consistent with the methyl radical-based mechanism, there was no evidence for a methyl-Ni(III) species; furthermore, magnetic circular dichroism spectroscopy identified the Ni(II)-thiolate intermediate. Temperature-dependent transient kinetics also closely matched density functional theory predictions of the methyl radical mechanism. Identifying the key intermediate in methanogenesis provides fundamental insights to develop better catalysts for producing and activating an important fuel and potent greenhouse gas.

In this work we investigate the ability of the uracil‚water complex to form stable anionic systems. As the experimental evidence and theoretical calculations have indicated, the isolated uracil molecule can only attach an excess electron into a diffuse dipole-bound state, while some recent experiments suggest that the uracil‚ water complex can form a more stable valence-type anion. In this work we demonstrate that it is possible to converge ab initio calculations of uracil‚(H 2 O) 3 -to an equilibrium structure that is significantly different from the structure of the neutral cluster and that has a positive and remarkably significant vertical ionization potential. Apart from the valence anion, the uracil‚(H 2 O) 3 complex can form a stable dipolesbound anion, but as the present calculations indicate the electron affinity, which corresponds to this attachment, is very small (13 meV). The structure of the dipole-bound anion is virtually identical with the structure of the neutral complex.

The influence of N-methylation on the dipole-bound electron affinities of pyrimidine nucleic acid bases, uracil and thymine, has been investigated theoretically using ab initio quantum mechanical calculations, and experimentally using Rydberg electron transfer spectroscopy. Both experiment and theory are consistent in showing that replacement of hydrogen atoms by methyl groups reduces electron affinities corresponding to formation of dipole-bound anions of these systems. Also, the distortion of the anion geometries with respect to the geometries of the neutral parents are reduced with the methylation.