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Abstract
Density cumulant theory (DCT) describes the state of an electronic system byvariationally optimizing a parametrization of the two-particle density cumulant.The cumulant is a statistical descriptor of the wavefunction distribution whichnaturally decouples independent subsystems, leading to desirable properties likesize-extensivity which have given the coupled-cluster methods pride of place inelectronic structure theory.We present benchmark calculations demonstrating the superior performance ofdensity cumulant theory relative to coupled-cluster theory for the descriptionof ground-state properties.Next, we extend this method for the description of excited states using linearresponse theory.Finally, we develop algorithms for the new excited state model which enable usto study larger systems.