Occupation restricted multiple active spaces

More precisely, a set of active orbitals and a number of active electrons are defined. All the configurations that can be built by excitations ofthe active electrons among the active orbitals are included into the wavefunction. Other, more flexible approaches for the selection of the conflgurations included in the MSCSF wavefunction exist, such as the Restricted Active Space Self-Consistent Field [23, 24] (RASSCF) and the Occupation Restricted Multiple Active Space (ORMAS) [25] methods. 

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