  We present a mean-field approach for accurately describing strong
correlations via electron number fluctuations and pairings constrained to an
active space. Electron number conservation is broken and correct only on
average but both spin and spatial symmetries are preserved. Optimized natural
orbitals and occupations are determined by diagonalization of a mean-field
Hamiltonian. This constrained-pairing mean-field theory (CPMFT) yields a
two-particle density matrix ansatz that exclusively describe strong
correlations. We demonstrate CPMFT accuracy with applications to the
metal-insulator transition of large hydrogen clusters and molecular
dissociation curves.

Materials Science;Strongly Correlated Electrons