When a chemical bond breaks, a pair of radicals (species with unpaired electrons) is usually formed. That is why the mechanisms of many chemical reactions involve open-shell species as transient intermediates or transition states. Photosynthesis and vision, the interaction of anticancer drugs with tumor cells, enzyme catalysis, combustion, and the atmospheric ozone production/decomposition cycles are just a few examples. Unraveling the mechanisms of such reactions requires understanding the electronic structure of open-shell species, a subject of an acute interest for many diverse experimental and computational groups.

Electronic structure calculations are indispensable in all areas of modern chemistry—the Royal Swedish Academy of Sciences has awarded the 1998 Nobel Prize in Chemistry to Prof. Walter Kohn (for his development of the density-functional theory) and Prof. John A. Pople. For example, the design and interpretation of experiments are often aided by high-level ab initio calculations of structural, thermochemical, and spectroscopic properties. Additional insight can be derived from the examination of wave functions and electron distributions, which are produced in electronic structure calculations. The predictive power of theory becomes even more important when open-shell or electronically excited species are involved. Indeed, the transient and highly reactive nature of radicals makes their experimental studies difficult, and their non-classical electronic structure challenges existing bonding paradigms. That is why theoretical support is crucial for experimentalists who encounter open-shell and electronically excited species in their research. However, inherent complexity of open-shell wave functions and ensuing electronic structure methodology creates stumbling blocks that prevent the experimentalist to take full advantage of the state-of-the-art ab initio tools. To address this problem, the Center provides training and access to the advanced ab initio software to several experimental groups who encounter open-shell and electronically excited species in their research.

The Center's research activities focus on the development of:

• interfaces between electronic structure and spectroscopy.
• graphic visualization and interpretation tools (WebMO).
• distance learning tools and a streaming (i.e., video) digital library of electronic structure (BioSIGHT).

Our recent results include:

• development and implementation of Dyson orbitals within EOM-CC formalism and methodology for calculating agular distributions of photoelectrons for the ionization from the ground and electronically excited states;
• developing tools for calculating Franck-Condon factors.