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This is, of course, essential if one is trying to compute energy changes (bond energies and energies of formation) for chemical reactions. , twe beryllium atoms) energies evaluated separately within the same method. , doubly excited for Be2) could indeed yield a smooth potential-energy curve free of obvious pathological behavior. , the 2si2sfi configuration would dominate). The size consistency problem may be less significant if an appropriate configuration selection is performed at each geometry on the molecular potential surface, but the problem still remains as to how to efficiently choose configurations that describe equally well an entire potential energy surface.

The . diagonal and olT-diagonal matrix elements I 26 2 Energy and Wauifunction Optimization Methods of the Hamiltonian in {he residual space {In)} may, however, all be nonvanishing. 40) Eo) becomes diagonal. These simplifications remain in each step of the iterative process because a,CI calculation is performed in each iteration. Except for A22 and V, the form of the matrix element in the one- and iwo-step procedures are the same. The matrix elements of W, A11, and 811 may be derived erom Eqs. 42) by index substitution.

109)can be written as follows: " V. 115) Using these results in Eq. 117) By writing out the elements of V and Ho in terms of the integrals, we see that the iterative scheme for the evaluation of C and E can be written entirely in terms of sum s over integrals and c are brought into the core memory ofthe computer, all contributions of each successive integral to all of the sums appearing in Eqs.