A New Half‐Projection‐Based Treatment for Wave Function Optimization Within the Unrestricted Doubly Occupied Configuration Interaction Framework

ABSTRACT The unrestricted doubly occupied configuration interaction methods have enabled to describe satisfactorily ‐electron systems, although they very often yield spin‐contaminated wave functions. This contamination can partially be reduced by application of the half‐projection technique [J. Garcia et al. J. Chem. Phys. 161 , 224 112 (2024)]. In this work, we propose a new alternative treatment within the half‐projection scheme, which leads to results in terms of potential energy curves and their corresponding expectation values in prototype strongly correlated ‐electron systems. These results have been systematically compared with those obtained from previously reported treatments using half‐projection techniques, with those of the ordinary unrestricted doubly occupied configuration interaction procedure, and with those arising from the full configuration interaction method. We show that the new proposed method allows for a further reduction in the spin contamination, while it does not require significant increases in computational costs.