title: Constrained active space unrestricted mean-field methods for controlling spin-contamination.
abstract: We have recently proposed a novel approach for obtaining high-spin restricted open-shell Hartree-Fock wave functions by imposing constraints on the unrestricted Hartree-Fock (UHF) method [T. Tsuchimochi and G. E. Scuseria, J. Chem. Phys. 133, 141102 (2010)]. We here extend these ideas to the case where the constraints are released in an active space but imposed elsewhere. If the active space is properly chosen, our constrained UHF (CUHF) method greatly benefits from a controlled broken-symmetry effect while avoiding the massive spin contamination of traditional UHF. We also revisit and apply Löwdin's projection operator to CUHF and obtain multireference wave functions with moderate computational cost. We report singlet-triplet energy splittings showing that our constrained scheme outperforms fully unrestricted methods. This constrained approach can be readily used in spin density functional theory with similar favorable effects.
authors.affiliation: Department of Chemistry, Rice University, Houston, Texas 77005-1892, USA.
authors.forename: Takashi
authors.initials: T
authors.lastname: Tsuchimochi
authors.forename: Gustavo E
authors.initials: GE
authors.lastname: Scuseria