  Current Non-Orthogonal 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 paper,
we propose a method of applying spin rotation on symmetry broken UHF (sb-UHF)
states to generate a basis for NOCI. The dissociation of ethene was examined by
localising 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 Localised Molecular
Orbitals (ALMO). Furthermore, we demonstrate that the method can be used to
restore spin symmetry of symmetry broken SCF wavefunctions for molecules of
various sizes, marking an improvement over existing NOCI methods.

Chemical Physics