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Active-space coupled-cluster methods

Active-space coupled-cluster (CC) methods, including their origin and recent developments, are reviewed, and further challenges in this area are briefly discussed. The focus is on the significance of the paper by Professors Stanisław A. Kucharski and Rodney J. Bartlett, and the author of the present article [J. Chem. Phys. 110, 6103 (1999)], written when the author of the present article was a member of Professor Rodney J. Bartlett's research group at Quantum Theory Project. It is argued that that paper has played an important role in bringing the idea of selecting higher-than-double excitations within the single-reference CC framework to mimic multi-reference theories, proposed earlier [N. Oliphant and L. Adamowicz, J. Chem. Phys. 94, 1229 (1991); P. Piecuch et al., J. Chem. Phys. 99, 1875 (1993)], to the forefront of quantum chemistry development work, while inspiring extensions of such methods to electronically excited states and valence systems around closed shells via the equation-of-motion CC theory.