[en] Developing the knowledge base needed to address the environmental restoration issues of the US Department of Energy requires a fundamental understanding of molecules and their interactions in insolation and in liquids, on surfaces, and at interfaces. To meet these needs, the PNL has established the Environmental and Molecular Sciences Laboratory (EMSL) and will soon begin construction of a new, collaborative research facility devoted to advancing the understanding of environmental molecular science. Research in the Theory, Modeling, and Simulation program (TMS), which is one of seven research directorates in the EMSL, will play a critical role in understanding molecular processes important in restoring DOE's research, development and production sites, including understanding the migration and reactions of contaminants in soils and groundwater, the development of separation process for isolation of pollutants, the development of improved materials for waste storage, understanding the enzymatic reactions involved in the biodegradation of contaminants, and understanding the interaction of hazardous chemicals with living organisms. The research objectives of the TMS program are to apply available techniques to study fundamental molecular processes involved in natural and contaminated systems; to extend current techniques to treat molecular systems of future importance and to develop techniques for addressing problems that are computationally intractable at present; to apply molecular modeling techniques to simulate molecular processes occurring in the multispecies, multiphase systems characteristic of natural and polluted environments; and to extend current molecular modeling techniques to treat complex molecular systems and to improve the reliability and accuracy of such simulations. The program contains three research activities: Molecular Theory/Modeling, Solid State Theory, and Biomolecular Modeling/Simulation. Extended abstracts are presented for 89 studies

[en] Standard and augmented correlation consistent sextuple zeta (cc-pV6Z and aug-cc-pV6Z) basis sets have been determined for the second-row atoms aluminum through argon. Using these sets, dissociation energies and spectroscopic constants for the ground states of HCl, PN, and P₂ have been calculated using several theoretical methods, including Moeller-Plesset perturbation theory, coupled cluster theory, and multireference configuration interaction theory (MRCI). The aug-cc-pV6Z and cc-pV6Z sets yield dissociation energies that are estimated to be within 0.1--0.2 kcal/mol of the complete basis set limit for HCl and within 1--1.5 kcal/mol for PN and P₂. The MRCI and CCSD(T) methods are found to give the most consistently reliable results for the spectroscopic constants of all three species investigated. Use of the counterpoise correction improves the convergence behavior of the spectroscopic constants with increasing n for both the cc-pVnZ and aug-cc-pVnZ sets and should allow more accurate estimates of the complete basis set limit to be predicted

[en] We report reaction paths for two prototypical chemical reactions: Li + HF, an electron transfer reaction, and OH + H₂, an abstraction reaction. In the first reaction we consider the connection between the energetic terms in the reaction path Hamiltonian and the electronic changes which occur upon reaction. In the second reaction we consider the treatment of vibrational effects in chemical reactions in the reaction path formalism. 30 refs., 9 figs

[en] The electronic structure of molecules is discussed and the techniques of electronic structure calculations presented. Without exception the molecular electronic wave functions are expanded in some convenient, but physically motivated set of one-electron functions. Since the computational effort strongly depends on the number of expansion functions, the set of functions must be limited as far as possible without adversely affecting the accuracy of the wave functions. The choice of such functions is discussed for molecular calculations. Some particular molecules including hydrogen are considered. 28 references

[en] For a full configuration interaction (CI) calculation the choice of orbitals is completely irrelevant, i.e., the calculated wavefunction is unaffected by an arbitrary unitary transformation of the orbitals; it depends only on the space spanned by the original basis set. For most chemical systems it is not possible to realistically carry out a full CI calculation, so that specification of the orbital set is important. Even for less-than-full CI calculations, it can be shown, however, that for certain types of calculations the wavefunction is unaffected by restricted transformations among the orbital set. For example, for CI calculations based on a single configuration plus a complete set of excitations of a given type (single, double, etc.), the calculated wavefunction is independent of transformations among the set of occupied orbitals and among the set of virtual orbitals. The wavefunction does, however, depend on transformations which mix the occupied and virtual orbitals

[en] This paper presents ab initio potential surface parameters and transition state theory (TST) rate constants for the reaction H₂ + CH₃ → H + CH₄, its reverse, and all the deuterium isotopic counterparts associated with it and its reverse. The potential surface parameters are derived from accurate POLCI calculations and include vibrational frequencies, moments of inertia, and other quantities for CH₃, H₂, CH₄, and the H-H-CH₃ saddle point. TST rate constants are calculated from standard expressions and the Wigner tunneling correction. For H₂ + CH₃ and H₂ + CD₃, agreement of the rate constant with experiment is good over a broad temperature range, suggesting that the calculated 10.7 kcal/mol barrier is accurate to within about 0.5 kcal/mol barrier. Agreement with experiment for H + CH₄ using the calculated 13.5 kcal/mol reverse reaction barrier is poorer; a 12.5 kcal/mol barrier is found to provide a more reasonable estimate of the true barrier. The analysis of isotope effects in the H + CH₄ reaction is restricted to examining the branching ratios between H and D atom abstraction in the reaction of H with the mixed species CH₃D, CH₂D₂, and CHD₃. A combination of reaction path multiplicity, favorable zero point energy shifts, and a greater likelihood of tunneling causes H atom abstraction to predominate over D atom abstraction in H + CH₃D and H + CH₂D₂, but for H + CHD₃, it was found that the H atom and D atom abstraction rate constants cross near 700 K, with H atom abstraction dominating at low temperatures and D atom at high. 9 figures, 7 tables

[en] From the late 1940s to the late 1980s, the Department of Energy (DOE) had a critical role in the Cold War. Many sites were built to contribute to the nation's nuclear weapons effort. However, not enough attention was paid to how the waste generated at these facilities should be handled. As a result, a number of sites fouled the soil around them or dumped low-level radioactive waste into nearby rivers. A DOE laboratory is under construction with a charter to help. Called the Environmental Molecular Sciences Laboratory (EMSL), this national user facility will be located at DOE's Pacific Northwest Laboratory (PNL) in Richland, WA. This laboratory has been funded by DOE and Congress to play a major role as the nation confronts the enormous challenge of reducing environmental and human risks from hundreds of government and industrial waste sites in an economically viable manner. The original proposal for the EMSL took a number of twists and turns on its way to its present form, but one thing remained constant: the belief that safe, permanent, cost-effective solutions to many of the country's environmental problems could be achieved only by multidisciplinary teams working to understand and control molecular processes. The processes of most concern are those that govern the transport and transformation of contaminants, the treatment and storage of high-level mixed wastes, and the risks those contaminants ultimately pose to workers and the public

[en] From the ozone hole and the greenhouse effect to plastics recycling and hazardous waste disposal, society faces a number of issues, the solutions to which require an unprecedented understanding of the properties of molecules. We are coming to realize that the environment is a coupled set of chemical systems, its dynamics determining the welfare of the biosphere and of humans in particular. These chemical systems are governed by fundamental molecular interactions, and they present chemists with an unparalleled challenge. The application of current concepts of molecular behavior and of up-to-date experimental and computational techniques can provide us with insights into the environment that are needed to mitigate past damage, to anticipate the impact of current human activity, and to avoid future insults to the environment. Environmental chemistry encompasses a number of separate, yet interlocking, areas of research. In all of these areas progress is limited by an inadequate understanding of the underlying chemical processes involved. Participation of all chemical approaches -- experimental, theoretical and computational -- and of all disciplines of chemistry -- organic, inorganic, physical, analytical and biochemistry -- will be required to provide the necessary fundamental understanding. The Symposium on ''Physical Chemistry and the Environment'' was designed to bring the many exciting and challenging physical chemistry problems involved in environmental chemistry to the attention of a larger segment of the physical chemistry community

[en] A comprehensive theoretical study has been made of the energetics of the important pathways involved in the reaction of hydrogen atoms with hydrogen cyanide. For each reaction ab initio GVB-CI calculations were carried out to determine the structures and vibrational frequencies of the reactants, transition states, and products; then POL-CI calculations were carried out to more accurately estimate the electronic contribution to the energetics of the reactions. The hydrogen abstraction reaction is calculated to be endoergic by 24 kcal/mol [expt. ΔH (0 K) = 16--19 kcal/mol] with a barrier of 31 kcal/mol in the forward direction and 6 kcal/mol in the reverse direction. For the hydrogen addition reactions, addition to the carbon atom is calculated to be exoergic by 19 kcal/mol with a barrier of 11 kcal/mol, while addition to the nitrogen center is essentially thermoneutral with a barrier of 17 kcal/mol. Calculations were also carried out on the isomerization reactions of the addition products. The cis→trans isomerization of HCNH has a barrier of only 10 kcal/mol with the trans isomer being more stable by 5 kcal/mol. The (1,2)-hydrogen migration reaction, converting H₂CN to trans-HCNH, is endoergic by only 14 kcal/mol, but the calculated barrier for the transfer is 52 kcal/mol. The energy of the migration pathway thus lies above that of the dissociation--recombination pathway

[en] Ab initio POL--CI calculations, augmented by a dispersion term, are used to predict the potential energy surface for the reaction Cl+HCl. The saddle point is found to be nonlinear. The surface is represented by a rotated-Morse-oscillator spline fit for collinear geometries plus an analytic bend potential. Variational transition state theory calculations, based on a linear reference path, are carried out, and they yield much smaller rate constants than conventional transition state theory, confirming that earlier similar results for this heavy--light--heavy mass combination were consequences of the small skew angle and were not artifacts of the more approximate potential energy surfaces used in those studies. Transmission coefficients are calculated using approximations valid for large-reaction-path curvature and the potential along the reference path is scaled so that the calculated rate constant agrees with experiment. The resulting surface is used to compute the H/D kinetic isotope effect which is in qualitative agreement with experiment