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
+ (*n*^{2}(*n*-1).*m*^{2}(*m*-1))/2 + (*n*(*n*-1)(*n*-2).*m*(*m*-1)(*m*-2))/18

This represents:

Ground state

+ (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.