Node 1: Theoretical and computational modelling: structure and scattering

The covering theory of all of the phenomena considered by XENQ is relativistic quantum electrodynamics (QED). At this fundamental level, experimental and theoretical investigations of the validity of QED in the limit of strong external fields continue to challenge our understanding of the physical world, and of the electrodynamic forces that determine the physical properties of atoms, molecules, clusters, and solids. Even apparently routine studies of inner-shell processes, such as X-ray emission spectroscopy, are sensitive to a number of quantum electrodynamic effects, including the Lamb shift in the energy levels of core electrons, and the magnetic energy perturbations mediated by the Breit interaction. Many of the most interesting systems involve heavy elements (lanthanides, actinides and third row transition metals), introducing a subtle interplay between relativistic and quantum electrodynamical effects.

While the procedural basis of QED is well-established, and has been developed to a high art for simple model systems in the weak-field limit, the implementation of accurate ab initio relativistic methods for both structural and dynamical studies of heavy element systems remains a challenge to theory, and has been pursued actively by Quiney (Melbourne). This has led to the development of a practical implementation of bound-state QED that is sufficiently general that it is able to model the electronic structures of atoms, molecules, clusters and solids. However, this scheme borrows sufficient resources from the prevailing non-relativistic, semi-classical models that it is able to exploit the existing investment in algorithm design, and to interface seamlessly with the more familiar approaches of atomic and molecular physics and quantum chemistry.

Of course, one need not wield the full power of relativistic QED in every application, and a range of approximate methods to model heavy element electronic structures and the probing interaction of these systems with photons or electrons has also been developed. In the limit of light element systems, a wide range of commercially available software is available that is able to generate theoretical models of high accuracy, though the black-box nature of many of these packages often leads the inexperienced user to generate misleading results. We envisage that a strong interplay between theoretical and computational communities within this node will raise the quality of structural and dynamical modelling within the XENQ community and will contribute greatly to the resolution of problems and the development of new, reliable directions of experimental enquiry.

To complement this wide range of essentially structural models, new theoretical methods, especially those of Bray and Stelbovics (Murdoch), have been developed to model processes involving continuum processes within the convergent close-coupled scheme. These have led to fundmental insights into the physics of photoionisation and electron-atom impact phenomena, two of the cornerstones of the XENQ network. The faithful extension of these methods to modelling continuum processes in molecules, solids, and clusters remains a formidable challenge to theorists, and a keenly awaited development amongst experimentalists. The current difficulties encountered in modelling accurately continuum processes in heavy element solids, for example, remains a major obstacle in a detailed theoretical understanding of anomalous fine structure in near-edge X-ray absorption spectroscopy, which is widely used both in structural analysis and in the application of biomedical therapies.

This structure of this node is configured to chart a landscape with QED, the most extensively tested and intellectually exquisite of all the physical theories at its centre. On the horizon lies computational modelling and physical insights into the many applications of emerging electronic and photonic technologies in the development of new medical treatments, and new advanced materials.