$\mathbb{i}$CAS: Imposed Automatic Selection and Localization of Complete Active Spaces

Abstract

It is shown that, in the spirit of "from fragments to molecule" for localizing molecular orbitals [J. Chem. Theory Comput. 7, 3643 (2011)], a prechosen set of occupied/virtual valence/core atomic/fragmental orbitals can be transformed to an equivalent set of localized occupied/virtual pre-molecular orbitals (pre-LMO), which can then be taken as probes to select the same number of maximally matching localized occupied/virtual Hartree-Fock (HF) or restricted open-shell Hartree-Fock (ROHF) molecular orbitals as the initial local orbitals spanning the desired complete active space (CAS). In each cycle of the self-consistent field (SCF) calculation, the CASSCF orbitals can be localized by means of the noniterative "top-down least-change" algorithm for localizing ROHF orbitals [