Elements: NSCI-Software -- A General and Effective B-Spline R-Matrix Package for Charged-Particle and Photon Collisions with Atoms, Ions, and Molecules

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This project concerns the development and subsequent distribution of a suite of computer codes that can accurately describe the interaction of charged particles (mostly electrons) and light (mostly lasers and synchrotrons) with atoms and ions. The results are of importance for the understanding of fundamental collision dynamics, and they also fulfill the urgent practical need for accurate atomic data to model the physics of stars, plasmas, lasers, and planetary atmospheres. With the rapid advances currently seen in computational resources, such studies can now be conducted for realistic systems, as opposed to idealized models. In particular, it has become possible to describe very complex targets, such as transition metals and other open-shell systems. Examples include the excited states of the inert gases beyond helium, as well as neutral and lowly-ionized iron. These systems are of significant importance for plasma diagnostics and astrophysics, respectively. The source code will be made publicly available. The project will support a post-doctoral researcher. A website devoted to user-developer interaction will be developed and maintained together with the neccessary code documentation and training materials.The numerical calculations will be based upon the non-perturbative R-matrix (close-coupling) method. A particular strength of the implementation pursued in this project is the use of a highly flexible B-spline basis with non-orthogonal orbital sets. The major advantage of the approach compared to traditional methods is the fact that an accurate target description can be achieved with a much-reduced configuration-interaction (CI) expansion if the orthogonality requirements on the individual orbitals are relaxed. This is critical for complex targets, where the valence orbitals in particular are known to be strongly term-dependent. Using a sufficiently small but still accurate enough CI expansion for the target states is essential for the feasibility and quality of the subsequent collision calculation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.