We present a mean-field approach for accurately describing strong correlations via electron numberfluctuations and pairings constrained to an active space. Electron number conservation is broken andcorrect only on average, but both spin and spatial symmetries are preserved. Optimized naturalorbitals and occupations are determined by diagonalization of a mean-field Hamiltonian. Thisconstrained-pairing mean-field theory (CPMFT) yields a two-particle density matrix ansatz thatexclusively describes strong correlations. We demonstrate CPMFT accuracy with applications to themetal-insulator transition of large hydrogen clusters and molecular dissociation curves.
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