Localized Spin Rotations: A Size-Consistent Approach to Nonorthogonal Configuration Interaction

Current nonorthogonal configuration
interaction (NOCI) methods
often use a set of self-consistent field (SCF) states selected based
on chemical intuition. However, it may be challenging to track these
SCF states across a dissociation profile and the NOCI states recovered
may be spin contaminated. In this Article, we propose a method of
applying spin rotation on symmetry broken unrestricted Hartree–Fock
(sb-UHF) states to generate a basis for NOCI. The dissociation of
ethene was examined by localizing spin rotation on each resulting
carbene fragment. We show that this gives a size-consistent description
of its dissociation and results in spin-pure states at all geometries.
The dissociation was also studied with different orbitals, namely,
canonical UHF and absolutely localized molecular orbitals (ALMO).
Furthermore, we demonstrate that the method can be used to restore
spin symmetry of symmetry broken SCF wave functions for molecules
of various sizes, marking an improvement over existing NOCI methods.