We introduce an electronic structure approach for spin-symmetry breaking and restoration at the mean-field level. The spin-projected constrained-unrestricted Hartree–Fock (SPcUHF) method restores the broken spin symmetry inherent in spin-constrained-UHF determinants by employing a nonorthogonal configuration interaction (NOCI) projection method. This method includes all possible configurations in spin space compatible with a Clebsch–Gordon recoupling scheme in a NOCI calculation. The tunable one-pair-at-a-time characteristics of the symmetry-breaking process in c-UHF allow us to reduce the computational costs of full projection. SPcUHF is tested on 4-, 6-, and 8-electron systems that exhibit dominant static and/or dynamic correlations.

The uniform electron gas (UEG) is a cornerstone of density-functional theory (DFT) and the foundation of the local-density approximation (LDA), one of the most successful approximations in DFT. In thi...