We present a mean-field approach for accurately describing strongcorrelations via electron number fluctuations and pairings constrained to anactive space. Electron number conservation is broken and correct only onaverage but both spin and spatial symmetries are preserved. Optimized naturalorbitals and occupations are determined by diagonalization of a mean-fieldHamiltonian. This constrained-pairing mean-field theory (CPMFT) yields atwo-particle density matrix ansatz that exclusively describe strongcorrelations. We demonstrate CPMFT accuracy with applications to themetal-insulator transition of large hydrogen clusters and moleculardissociation curves.
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