Method for including static correlation in molecules

New ways to treat electron correlation in electronic structure problems are discussed in the context of many-electron theory. The present work focuses primarily on static correlation. In related work, a method for including dynamical correlation effects is described. The overlap density of two basis functions i, j and the associated density matrix is a signature of bond formation and can be used to define a local molecular orbital, i + j. The total electron density \r{ho} can be written in terms of densities derived from these two-center orbitals and residual one-center terms. In the interaction of total densities, the self-energy terms resulting from an average field (Hartree-Fock) Hamiltonian are allowed to respond to an explicit inclusion of electron repulsion by mixing (i + j)1(i + j)2 +λ(i - j)1(i - j)2 . The energy lowering weighted by the density matrix ij approximates this contribution to the correlation energy of the system. Numerical calculations for a set of 20 molecules representing different bonding environments are reported and results are compared with configuration interaction calculations using the same molecular orbital basis. Calculations on chlorin, N4C20H16, are reported as an example of how the method could be used in an embedding treatment of a large system.