RHF interpretation: the desired spin-state is a quartet, i.e., the state with component of spin MS = 3/2 and spin S = 3/2. In order to define this type of calculation other keywords must also be used. For a 'simple' quartet calculation, use C.I.=3. If the quartet state consists of three half-filled degenerate M.O.s, then OPEN(3,3) should be used.
When the configuration interaction calculation is done, all microstates having a component of spin, MS, equal to 3/2 are selected. These microstates are then used in the construction of states. Because of the way in which the microstates were chosen, only states of spin equal to or greater than 3/2 can be constructed. From this set, only a quartet state can be selected; all other states will be ignored. If ROOT=n is present, then the n'th quartet state will be selected; otherwise, the first quartet state will be chosen. See also SINGLET, DOUBLET, TRIPLET, QUINTET, SEXTET, SEPTET, OCTET, and NONET. UHF interpretation: The system will have three more alpha electrons than beta electrons.