Classification of configurations and the determination of interacting and noninteracting spaces in configuration interaction

The n-particle space of a configuration interaction (CI) calculation, spanned by configuration state functions (CSF's), is partitioned into subspaces defined, relative to a set of zeroth-order CSF's, by the type of orbital excitation and by the lowest order of the Rayleigh-Schrödinger perturbative correction contributed by the subspace. A method is given for determining CSF's spanning the ith-order subspace which contains those and only those functions that have a nonzero interaction through the Hamiltonian with some member of the (f- l)th-order subspace. These subspaces are anticipated to be of considerable use in structuring and interpreting configuration interaction wavefunctions.