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.

This paper describes capabilities, evolution, performance, and applications of the Global Arrays (GA) toolkit. GA was created to provide application programmers with an inteface that allows them to distribute data while maintaining the type of global index space and programming syntax similar to that available when programming on a single processor. The goal of GA is to free the programmer from the low level management of communication and allow them to deal with their problems at the level at which they were originally formulated. At the same time, compatibility of GA with MPI enables the programmer to take advatage of the existing MPI software/libraries when available and appropriate. The variety of applications that have been implemented using Global Arrays attests to the attractiveness of using higher level abstractions to write parallel code.

A method for generating extended X-ray absorption fine-structure (EXAFS) spectra directly from molecular dynamics trajectories is described. Conventional molecular dynamics trajectories are run, and configurations from the trajectory are saved at periodic intervals. Clusters containing the EXAFS atom at the center are extracted from the configurations and used as input to the multiple scattering code FEFF6, which produces an EXAFS spectrum for each cluster. The separate spectra for the individual clusters are then averaged together to get a configurationally averaged EXAFS spectrum that can be compared directly with experiment. The method is tested on strontium chloride solutions and compared with recent experimental results on strontium nitrate solutions at both standard and supercritical conditions.

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X-ray absorption fine structure (XAFS) spectroscopy was used to measure the coordination structure about Cu2+, Cu1+, and Br- in water at temperatures up to 325 °C. The hexaaqua Cu2+ species maintains its distorted octahedral structure up to 325 °C, whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu1+ species, [CuBr2]-, is predominant at 200 °C and above for systems having excess Br-. Even for a very high salt concentration of 2.0 m NaBr, only the dibromo Cu1+ species, [CuBr2]-, is observed with no evidence of higher Br- coordination. For this dibromo-species there are no tightly bound hydration waters in the first shell. In the absence of excess Br-, a monoaqua monobromo Cu1+ species, [Cu(H2O)Br] is observed. For certain systems, both Cu and Br XAFS were acquired, and a global model was used to fit the two independent sets of XAFS data. Thus, the results represent a complete picture of the coordination structure about Cu1+ including the coordination numbers, distances for the ion−ion and water-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye−Waller factor and the anharmonicity) of the Cu1+/Br- association. Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr2]- species under hydrothermal conditions. We found quantitative agreement for the Cu−Br interactions, but the simulation has difficulty predicting the experimental Cu−H2O interaction. In particular, the amount of scattering from the water in the dibromo Cu1+ complex was highly over-predicted, so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.