The number of states considered in a CISDT 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 + (n2(n-1).m2(m-1))/2 + (n(n-1)(n-2).m(m-1)(m-2))/18
+ (one α electron excited + one β electron excited)
+ (one α electron and one β electron excited)
+ (two α electrons excited + two β electrons excited)
+ (one α electron and two β electrons excited + two α electrons and one β electron excited)
+ (three α electrons excited + three β electrons excited)
The first term represents the ground state, the second term represents number of one-electron excitations, the third and fourth terms represent the number of two-electron excitations, and the fifth and sixth terms represent the number of three-electron excitations.