In configuration interaction calculations, the ground state and microstates resulting from single electron excitations are used if CIS is specified. (Read CIS as Configuration Interaction Singles.)

In a configuration interaction involving an active space in which n levels are doubly occupied and m levels are empty, the number of states considered in the C.I. would be (1 + 2.n.m).  By Koopmans' theorem, the ground microstate does not interact with any microstates resulting from one-electron excitation; this means that the lowest state resulting from the C.I. consists of the doubly occupied microstate only. It is included in the C.I. so that the relative energies are relative to the ground state.  Because the C.I. cannot affect the lowest root, a CIS ground-state calculation should not be used in runs that affect the geometry, e.g. geometry optimization or vibrational frequency calculations.