A new symmetric group program for direct configuration interaction studies of molecules

A symmetric group approach is presented for performing large scale, direct configuration interaction (CI) studies of electronic correlation effects in molecules. Two alternative graphical methods are described for representing very large sets of lexically ordered orbital configurations. One is suited for truncations in the number of core orbitals and the other for changes in the number of virtual orbitals. The Hamiltonian matrix elements are constructed using a graphical approach for localizing the one-and two-electron contributions. Compact and direct algorithms are given for calculating the irreducible representation matrices of S(N) for the “line-up” permutations. The program implementation of the CI procedure, PEDICI, calculates directly firstorder molecular one-electron properties and transition probabilities from a multireference space for general excitation schemes.