Size-Nonextensive Contributions in Singles-Only CI

The size-nonextensivity (SNE) of the configuration interaction scheme with single excitations only (CIS) is discussed. On the basis of model considerations the method is explicitly shown to give energies which have wrong dependence on the number of particles. The same model of a multimer of N noninteracting subsystems, each in the same singly excited state, is analyzed in terms of the many-body perturbation theory expansion. This suggests new computational schemes which will be either completely free of the size-nonextensive contributions or may have them removed a posteriori. The extension of the perturbation scheme leads to the singles-only coupled cluster approach with a singly excited reference function. The consequences of the present model study for computational methods for inexpensive calculations of the electronic spectra of molecules, are investigated.