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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.

We applied our recently developed protocol of the conductorlike continuum model of solvation to describe the title reaction in aqueous solution. The model has the unique feature of the molecular cavity being dependent on the atomic charges in the solute and can be extended naturally to transition states and reaction pathways. It was used to calculate the reaction energetics and reaction rate in solution for the title reaction. The rate of reaction calculated using canonical variational transition state theory in the context of the equilibrium solvation path approximation, and including correction for tunneling through the small curvature approximation, was found to be 3.6 x 10(6) M(-1) s(-1), significantly slower than in the gas phase in accord with experiment. These results suggest that the present protocol of the conductorlike continuum model of solvation with the charge-dependent cavity definition captures qualitatively and quantitatively the solvation effects at transition states and allows for quantitative estimates of reaction rates in solutions.

The mechanism of the hydrogen abstraction reaction H(2)O(2)+OH-->HO(2)+H(2)O in gas phase was revisited using density functional theory and other highly correlated wave function theories. We located two pathways for the reaction, both going through the same intermediate complex OH-H(2)O(2), but via two distinct transition state structures that differ by the orientation of the hydroxyl hydrogen relative to the incipient hydroperoxy hydrogen. The first two excited states were calculated for selected points on the pathways. An avoided crossing between the two excited states was found on the product side of the barrier to H transfer on the ground state surface, near the transition states. We report on the calculation of the rate of the reaction in the gas phase for temperatures in the range of 250-500 K. The findings suggest that the strong temperature dependence of the rate at high temperatures is due to reaction on the low-lying excited state surface over a barrier that is much larger than on the ground state surface.

We present an approach to calculate the free energy profile along a condensed-phase reaction path based on high-level electronic structure methods for the reactive region. The bulk of statistical averaging is shifted toward less expensive descriptions by using a hierarchy of representations that includes molecular mechanics, density functional theory, and coupled cluster theories. As an application of this approach we study the reaction of CHCl3 with OH- in aqueous solution.

The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between the rigid OH radical and H(2)O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there is a low-lying excited state for the H(2)O.OH complex, the impact of the excited state on the chemical behavior of the OH radical can be very important. We perform a potential energy surface scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole-type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H(2)O for both ground and excited states. The stationary points of (H(2)O)(n)OH clusters were identified as a benchmark of the potential.

Quasiclassical ab initio simulations of the ionization dynamics in a (H(2)O)(17) cluster, the first water cluster that includes a fourfold coordinated (internally solvated) water molecule, have been carried out to obtain a detailed picture of the elementary processes and energy redistribution induced by ionization in a model of aqueous water. General features observable from the simulations are the following: (i) well within 100 fs following the ionization, one or more proton transfers are seen to take place from the "ionized molecule" to neighboring molecules and beyond, forming a hydronium ion and a hydroxyl radical; (ii) two water molecules close to the ionized water molecule play an important role in the reaction, in what we term a "reactive trimer." The reaction time is gated by the encounter of the ionized water molecule with these two neighboring molecules, and this occurs anytime between 10 and 50 fs after the ionization. The distances of approach between the ionized molecule and the neighboring molecules indeed display best the time characteristics of the transfer of a proton, and thus of the formation of a hydronium ion and a OH radical. These findings are consistent with those for smaller cyclic clusters, albeit the dynamics of the proton transfer displays more varieties in the larger cluster than in the small cyclic clusters. We used a partitioning scheme for the kinetic energy in the (H(2)O)(17) system that distinguishes between the reactive trimer and the surrounding "medium." The analysis of the simulations indicates that the kinetic energy of the surrounding medium increases markedly right after the event of ionization, a manifestation of the local heating of the medium. The increase in kinetic energy is consistent with a reorganization of the surrounding medium, electrostatically forced in a very short time by the water cation and in a longer time by the formation of the hydronium ion.

Transport of conduction electrons and holes through the lattice of alpha-Cr2O3 (chromia) is modeled as a valence alternation of chromium cations using ab initio electronic structure calculations and electron-transfer theory. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus' theory. The calculation of the electronic coupling followed the generalized Mulliken-Hush approach using the complete active space self-consistent-field (CASSCF) method and the quasidiabatic method. Our findings indicate that hole mobility is more than three orders of magnitude larger than electron mobility in both (001) and [001] lattice directions. The difference arises mainly from the larger internal reorganization energy calculated for electron-transport relative to hole-transport processes while electronic couplings have similar magnitudes. The much larger hole mobility versus electron mobility in alpha-Cr2O3 is in contrast to similar hole and electron mobilities in hematite alpha-Fe2O3 previously calculated. Our calculations also indicate that the electronic coupling for all charge-transfer processes of interest is smaller than for the corresponding processes in hematite. This variation is attributed to the weaker interaction between the metal 3d states and the O(2p) states in chromia than in hematite, leading to a smaller overlap between the charge-transfer donor and acceptor wave functions and smaller superexchange coupling in chromia. Nevertheless, the weaker coupling in chromia is still sufficiently large to suggest that charge-transport processes in chromia are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron-spin coupling within the Cr-Cr donor-acceptor pair, while the reorganization energy is essentially independent of the electron-spin coupling.

Transport of conduction electrons and holes through the lattice of alpha-Fe(2)O(3) (hematite) is modeled as a valence alternation of iron cations using ab initio electronic structure calculations and electron transfer theory. Experimental studies have shown that the conductivity along the (001) basal plane is four orders of magnitude larger than the conductivity along the [001] direction. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e., the reorganization energy and the electronic coupling matrix element that enter Marcus' theory. The calculation of the electronic coupling followed the generalized Mulliken-Hush approach using the complete active space self-consistent field method. Our findings demonstrate an approximately three orders of magnitude anisotropy in both electron and hole mobility between directions perpendicular and parallel to the c axis, in good accord with experimental data. The anisotropy arises from the slowness of both electron and hole mobilities across basal oxygen planes relative to that within iron bilayers between basal oxygen planes. Interestingly, for elementary reaction steps along either of the directions considered, there is only less than one order of magnitude difference in mobility between electrons and holes, in contrast to accepted classical arguments. Our findings indicate that the most important quantity underlying mobility differences is the electronic coupling, albeit the reorganization energy contributes as well. The large values computed for the electronic coupling suggest that charge transport reactions in hematite are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron spin coupling within the Fe-Fe donor-acceptor pair, while the reorganization energy is essentially independent of the electron spin coupling.

The reorganization energy is an important quantity controlling electron transfer rates. The internal contribution arising from the energy to reorganize donor/acceptor bonds can be evaluated by the "direct" and "4-point" methods. We examine how spatial separation leading to the noninteracting character of the donor and acceptor affects the reorganization energy. We show that the direct method captures contributions from interaction of the donor and acceptor while the 4-point method does not, and the two methods converge at large separation. Comparing reorganization energies determined by the two methods yields a measure of the degree of interaction between the initial and final states. The analysis is illustrated in the characterization of small polarons in iron oxides., ((c) 2004 American Institute of Physics.)