Further development of internally contracted multi-reference coupled-cluster theory

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Accurate first-principles methods are an important building block of modern quantum chemistry. They may serve for the reliable prediction of energies and properties of transient species or hazardous compounds and provide an internal standard for more approximate methods. The internally-contracted multireference coupled-cluster (icMRCC) theory developed by the applicant promises to transfer the high accuracy of standard coupled-cluster theory to systems with complex electronic structure, like polyradicals and transition metal compounds with open d-shells. In this project, we want to further improve the methodological foundations of the icMRCC theory and provide a new and efficient implementation that allows to take full advantage of the potential of the method. Specific developments are the investigation of pragmatic approximations to the full-blown icMRCC theory and a multistate extension, allowing to treat arbitrary excited states and quasi-degenerate cases. By using an embedding method that will also be further developed during this project, we will be able to apply the newly developed methods to large molecules, for instance, to realistic models for computing the energetics at reaction centers of catalysts or magnetic couplings of molecular magnets.