A rapidly convergent CI expansion based on several reference configurations, using optimized correlating orbitals

A method is presented for obtaining CI expansions, comprising all single and double excitations from an arbitrary set of reference configurations, in a rapidly convergent form. This rapid convergence results from the use of mutually nonorthogonal sets of correlating orbitals, and the method thus represents a generalization of the PNO–CI scheme. A computational implementation requires the following steps: generation of a configuration list and symbolic Hamiltonian matrix elements between configurations, optimization of correlating orbitals, transformation of required integrals, the construction of a numerical Hamiltonian matrix from these data, and the extraction of one or more roots from this matrix. Each of these steps is discussed in terms of the algorithm used in the present application. The results of illustrative calculations on the methylene and lithium monoxide molecules are given, together with observed computation times, and it is concluded that the present scheme should prove as useful for CI expansions based on several reference configurations as has the PNO–CI method for the simpler case of a single reference configuration.