CISDT

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

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

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)
+ (one a electron and two b electrons excited + two a electrons and one b electron excited)
+ (three a electrons excited + three b 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.