The state to be calculated is the first excited open-shell singlet state. If the ground state is a singlet, then the state calculated will be S(1); if the ground state is a triplet, then S(2). This state would normally be the state resulting from a one-electron excitation from the HOMO to the LUMO. Exceptions would be if the lowest singlet state were a biradical, in which case the EXCITED state could be a closed shell.
EXCITED is limited to restricted Hartree Fock systems, so UHF should not be present.
The EXCITED state will be calculated from a BIRADICAL calculation in which the second root of the C.I. matrix is selected. Abbreviation: EXCI.
Note: EXCITED is a redundant keyword, and represents a particular configuration interaction calculation. Experienced users of MECI can duplicate the effect of the keyword EXCITED by using the MECI keywords OPEN(2,2), SINGLET, and ROOT=2.