Abstract
A previously proposed perturbative variational CI method has been extended to the case of a many-dimensional reference space, where all the configurations have a common core plus two valence electrons. Compared to ordinary CI, the Hamiltonian and overlap matrices are extremely reduced in dimension. All matrix elements have been obtained by means of diagrammatic techniques, and a Fortran program adapted to medium–large orbital bases has been implemented. The method has been applied to ethylene in the ground state equilibrium geometry (D2h), giving about 97% of the total correlation energy of a SDCI calculation with the same AO basis. It has also been applied to 90° twisted ethylene (D2d), where a two-dimensional reference space is needed; the computed value of the ground state rotational barrier is about 67 kcal mol−1.
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