The active-space coupled-cluster (CC) and equation-of-motion (EOM) CC methods, in which higher-order components of the cluster and excitation operators are selected via active orbitals, represent the most straightforward way of incorporating multi-reference concepts within the CC framework. At the same time, the most natural way of describing electronic structure of radicals and biradicals, and other valence systems around closed shells is provided by the electron-attached (EA) and ionized (IP) EOMCC theories, and their multiply attached and multiply ionized extensions. This talk will discuss our recent progress in the EA/IP and active-space EOMCC methodologies, including the development of the doubly electron-attached (DEA) and doubly ionized (DIP) EOMCC theories with up to 4-particle-2-hole and 4-hole-2-particle excitations, and their inexpensive active-space variants. The discussion of the formal and algorithmic details will be augmented by the examples of benchmark calculations, including bond breaking and low-lying electronic states of biradicals, including singlet-triplet and singlet-singlet gaps.

Recent progress in the electron-attached, ionized, and active-space equation-of-motion coupled-cluster methodologies

245th National Meeting (2013)

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