Special Options for Open-Shell Systems

Flipping of spins at a selected atom is possible via define. This requires converged unrestricted HF (UHF) or unrestricted Kohn–Sham (UKS) molecular orbitals and no symmetry (C\(_1\)). Go to the molecular orbital section of defineand use the spin option. Note that this will localize the orbitals, assign them to the atoms and give the user the possibility to choose atoms at which alpha-orbitals are moved to beta orbitals, or vice versa. This is useful for spin-broken start orbitals, but not for spatial symmetry breaking.

If you encounter serious spin contamination in UHF or UKS calculations, you may use an additional spin constraint in the SCF procedure. This can be done with they keyword spin constraint followed by a value for the constraint. We refer to [84] and the Supporting Information of [85] for details.