Approximately extensive modifications of the multireference configuration interaction method: A theoretical and practical analysis

P. Szalay, Rodney J. Bartlett

Research output: Contribution to journalArticle


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) 2SCI]. 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 2O; 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 languageEnglish
Pages (from-to)3600-3612
Number of pages13
JournalPhysics of Fluids
Issue number11
Publication statusPublished - 1967


ASJC Scopus subject areas

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

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