Generalization of the direct configuration interaction method to the Hartree–Fock interacting space for doublets, quartets, and open‐shell singlets. Applications to NO2 and NO2−

The direct configuration interaction method, including all Hartree–Fock interacting single and double replacements of a single root function, has been generalized to a wide class of systems. Programs capable of handling closed‐shell singlets and open‐shell doublets, triplets, quartets, and singlets in a routine fashion are described. The open‐shell singlet presents some particular difficulties which have been overcome, and thus it appears that the direct CI method can be a fairly general and quite automatic approach to the electron correlation problem. The versatility of these programs is illustrated by applications to the vertical electronic spectra of NO2 (doublet and quartet spin states) and of NO2− (closed‐shell singlet, open‐shell singlet, and triplet spin states).