Final Report Summary - CCXSPEC (Coupled-cluster methods for theoretical X-ray spectroscopies) Synchrotron radiation provides highly-focused, high-intensity light of well-defined characteristics with wavelengths from infrared to X-ray, and is a widespread scientific tool in many areas of research including physics, chemistry, material science, and environmental sciences. A largely used synchrotron radiation technique is X-ray absorption spectroscopy, where the photon energy is tuned to regions corresponding to the excitation of core electrons. Its successful application relies not only on the development of high-quality radiation sources and experimental techniques but also on theoretical methodologies. The analysis of the core-level spectra in combination with theoretical calculations discloses detailed electronic and structural information, such as charge-transfer character of states, bonding nature, hybridization, chemical environment and site symmetry. Theoretical simulations are thus essential both to understand specific systems and to define the information content in the spectroscopic probes. The theoretical description of core-excitation spectra is however far from an easy task. The frequency region is very broad (20-50 eV) and requires the ability to treat a large number of excited states (approximately 50-500). Large relaxation effects occur upon excitation of a core electron, making a balanced treatment of the core-excited state relative to the ground state difficult. Strong excitations of charge-transfer character or of higher order may be induced. Relativistic effects are also important. All this poses stringent requirements on the quality and accuracy of the models employed to interpret the spectroscopic results. Even if a number of methodologies exist, often adapted to specific systems, elements or X-ray absorption edges, the development of a fully quantitative treatment remains challenging. Coupled-cluster (CC) response theory has proven to be 'the state-of-the-art method' for high-accuracy calculations of spectroscopic properties, with accuracy comparable, and sometimes even superior, to the one obtained in experimental studies. The hierarchy of coupled cluster levels enables systematic and rapid convergence of dynamic electron correlation, and, with inclusion of triple excitations, very high accuracy is reached in the description of electronic transitions in the UV/vis region of the spectrum. In X-ray spectroscopy, on the other hand, the use of CC response methods is almost unexplored, hampered mainly by the fact that the semi-bound core-valence excited states of interest are embedded in a continuum of valence-ionized states.To overcome this problem, and as an essential step towards the extension of CC theory to spectroscopic phenomena in the X-ray region, we have developed a new approach for a hierarchy of CC approximations where the absorption cross section is obtained not from the complete oscillator strength distribution, but instead from the imaginary part of the electric dipole polarizability. This is in the spirit of the 'damped' formulations of response theory, where one explicitly takes into account the finite lifetimes of the excited states. The approach has the same qualities and virtues for X-ray spectroscopies that traditional response function approaches have shown for optical spectroscopies. It properly accounts for relaxation effects and is open-ended towards extensions to other spectroscopies in the X-ray region, such as X-ray circular dichroism or multiphoton X-ray absorption. Its accuracy in the description of electronic relaxation and differential correlation for the core-excited state depends only on the description of dynamic correlation and can be monitored by using the CC hierarchy, as typically done for UV/vis spectra. Unlike other more approximate methods currently used to calculate X-ray absorption, there is a clear path to improved accuracy. The method is thus very useful also as a benchmark for standard approximate methods, i.e. TDDFT, and provides insight on extensions of these approximations. Another interesting future perspective is its combination with molecular mechanics methods for calculations on much larger systems, where the accuracy still can be maintained for a specific central core region.In broader terms, the scientific outcome both in terms of methodologies, related computer code and applicative results is contributing to increase both the applicant's and the host institution's scientific competitiveness and is favouring the establishment and strengthening of long-term synergies between theoreticians and experimentalists working on synchrotron-radiation phenomena and NLO. The project is having an impact on the development of network relations, collecting interested partners coming from both experimental and theoretical backgrounds, and from a variety of research fields, driving the growth of interdisciplinary research. Related documents Final Report - CCXSPEC (Coupled-cluster methods for theoretical X-ray spectroscopies)
