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.