A Rayleighndash;Schrouml;dinger multiple perturbation theory is presented in a general formulation useful for energy evaluations beyond the Hartreendash;Fock level. This theory allows one to computationally take advantage of the structure of the basis set. For example, threehyphen; and fourhyphen;centerhyphen;like integrals appearing in the second quantized form of the Hamiltonian may be treated as higher order perturbations. Explicit formulas for the wave function to second order and energy to fourth order are also diagrammatically derived. As examples the theory is applied to aromatic cyclic polyenes within the PPP approximation. A perturbation sequence is defined by the magnitude of the integrals. Within this model errors of less than 1 kcal/mol can be obtained even while neglecting 50percnt; of the integrals up to fourth order. Preliminary studies withabinitiocalculations show that even a greater percentage of two electron integrals can be neglected with little loss of accuracy when using this scheme.
展开▼
