title: Large systems at ab initio multireference level: a cheap treatment thanks to a division into fragments.
abstract: Thanks to the use of localized orbitals and the subsequent possibility of neglecting long-range interactions, the linear-scaling methods have allowed to treat large systems at ab initio level. However, the limitation of the number of active orbitals in a complete active space self consistent-field (CASSCF) calculation remains unchanged. The method presented in this paper suggests to divide the system into fragments containing only a small number of active orbitals. Starting from a guess wave function, each orbital is optimized in its corresponding fragment, in the presence of the other fragments. Once all the fragments have been treated, a new set of orbitals is obtained. The process is iterated until convergence. At the end of the calculation, a set of active orbitals is obtained, which is close to the exact CASSCF solution, and an accurate CASSCF energy can be estimated.
authors.affiliation: Laboratoire de Physique Quantique, Unité Mixte de Recherche (UMR) 5626 du Centre National de la Recherche Scientifique (CNRS), Institut de Recherche sur les Systemes Atomiques et Moléculaires Complexes (IRSAMC), Université Paul Sabatier, Toulouse, France.
authors.forename: Fabienne
authors.initials: F
authors.lastname: Bessac
authors.forename: Sophie
authors.initials: S
authors.lastname: Hoyau
authors.forename: Daniel
authors.initials: D
authors.lastname: Maynau