CISD

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

The number of states considered in a CISD C.I. involving n doubly-occupied M.O.s and m empty M.O.s is:

No. of States = 1 + 2.n.m  + (n.m)2 + (n(n-1).m(m-1))/2

This represents:

Ground state
+ (one a electron excited + one b electron excited)
+ (one a electron and one b electron excited)
+ (two a electrons excited + two b electrons excited)

The first term represents the ground state, the second term represents number of one-electron excitations, and the third and fourth terms represent the number of two-electron excitations.