### Abstract

The extensivity error of configuration interaction (CI) is well understood and unlinked diagram corrections must be applied to get reliable results. Besides the well known a posteriori Davidson-type corrections, several methods attempt to modify the CI equations a priori to obtain nearly extensive results, while retaining the convenience of working in a configuration space. Such unlinked diagram corrections are particularly important for multireference cases for which coupled-cluster (CC) calculations, which require a many-body, integral-based calculation, are more difficult. Several such multireference methods have been presented recently, ranging from the multireference linearized coupled cluster method (MR-LCCM), averaged coupled pair functional (MR-ACPF), through various quasidegenerate variational perturbation theory (QD-VPT), MR-coupled electron pair method (MR-CEPA) to size-consistent, self-consistent, selected CI [(SC) ^{2}SCI]. We analyze all of these methods theoretically and numerically, paying particular attention to the new multireference averaged quadratic CC method (MR-AQCC), and demonstrate its comparative quality of performance even when using small references spaces. We consider several demanding molecular examples that benefit from a multireference description, like bond stretching in H _{2}O; N _{2} and C _{2}; the insertion of Be into H _{2}; and the singlet-triplet splitting in CH _{2}. We also investigate the extensivity error.

Original language | English |
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Pages (from-to) | 3600-3612 |

Number of pages | 13 |

Journal | Physics of Fluids |

Volume | 10 |

Issue number | 11 |

Publication status | Published - 1967 |

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### ASJC Scopus subject areas

- Condensed Matter Physics
- Physics and Astronomy(all)
- Mechanics of Materials
- Computational Mechanics
- Fluid Flow and Transfer Processes

### Cite this

*Physics of Fluids*,

*10*(11), 3600-3612.