Some kinds of excited or higher ionized states can be calculated using a so-called
delta-SCF procedure. Here the excitation/ionization energy is
obtained as difference of the SCF solutions for the ground state and the
excited/ionized state. If the excited state is reasonably well described
by a single determinant, this procedure is normally quite accurate.
16.11 Advanced use: Core-excited states
shows how to use the delta-SCF procedure to calculate the N 1s core binding
energy in pyridine, by employing localized orbtials and moving them to
the HOMO position using a ROTATE card (Sec. 16.5). In
the core-excited state calculation, SCF is prevented from
collapsing to the ground state by using the option NITORD=1 (Sec. 16.1),
which asks SCF to lock onto the input solution.