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
+ (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.