A PARTIALLY RESTRICTED HARTREE-FOCK APPROACH

We present a Hartree-Fock method which gives the molecular orbitals (MOs) of a single determinant wave function consisting of a restricted part (one orbital assigned to each pair of spin-up and spin-down electrons) and an unrestricted part (different orbitals for different spins). The MOs of the restricted part; transform as irreducible representations of the point group of the molecule, whereas those of the unrestricted part may be symmetry broken. A modified coupling operator formalism is used for the determination of the MOs which result from different Fock operators for the closed shell orbitals (restricted part) and for the spin-up and spin-down orbitals of the unrestricted part. The method is illustrated by considering the symmetry breaking in the conjugated pi system of short-chain radical intermediates formed in the course of the solid state polymerization reaction of diacteylene crystals.