New scientific frontiers, recent advances in theory, and rapid increases in computational capabilities have created compelling opportunities for theory and computation to advance the scientific mission of the Office of Basic Energy Sciences (BES). The prospects for success in the experimental programs of BES will be enhanced by pursuing these opportunities. This report makes the case for an expanded research program in theory and computation in BES. The Subcommittee on Theory and Computation of the Basic Energy Sciences Advisory Committee was charged with identifying current and emerging challenges and opportunities for theoretical research within the scientific mission of BES, paying particular attention to how computing will be employed to enable that research. A primary purpose of the Subcommittee was to identify those investments that are necessary to ensure that theoretical research will have maximum impact in the areas of importance to BES, and to assure that BES researchers will be able to exploit the entire spectrum of computational tools, including leadership class computing facilities. The Subcommittee s Findings and Recommendations are presented in Section VII of this report.

The angular dependence of dissociative electron attachment (DEA) to polyatomic targets is formulated in the local complex potential model, under the assumption that the axial recoil approximation describes the dissociation dynamics. An additional approximation, which is found to be valid in the case of H2O but not in the case of H2S, makes it possible to describe the angular dependence of DEA solely from an analysis of the fixed-nuclei entrance amplitude, without carrying out nuclear dynamics calculations. For H2S, the final-vibrational-state-specific angular dependence of DEA isobtained by incorporating the variation of the angular dependence of the entrance amplitude with nuclear geometry into the nuclear dynamics. Scattering calculations using the complex Kohn method and, for H2S, full quantum calculations of the nuclear dynamics using the Multi-Configuration Time-Dependent Hartree method, are performed.

A numerically solvable two-dimensional model introduced by the authors [Phys. Rev. A 73, 032721 (2006)] is used to investigate the validity of the nonlocal approximation to the dynamics of resonant collisions of electrons with diatomic molecules. The nonlocal approximation to this model is derived in detail, all underlying assumptions are specified and explicit expressions for the resonant and non-resonant (background) T matrix for the studied processes are given. Different choices of the so-called discrete state, which fully determines the nonlocal approximation, are discussed and it is shown that a physical choice of this state can in general give poorer results than other choices that minimize the non-adiabatic effects and/or the background terms of the T matrix. The background contributions to the cross sections, which are usually not considered in the resonant theory of electron-molecule collisions, can be significant not only for elastic scattering but also for the inelastic process of vibrational excitation.

Excitation of the autoionizing states of helium by electron impact isshown in calculations in the s-wave limit to leave a clear signature in the singly differential cross section for the (e,2e) process. It is suggested that such behavior should be seen generally in (e,2e) experiments on atoms that measure the single differential cross section.