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

Publisher Website
Google Scholar
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)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 H2O; N2 and C2; the insertion of Be into H2; and the singlet–triplet splitting in CH2. We also investigate the extensivity error.