Treatment Of Ionized And Electron-attached States
Ionized states and electron-attached states of a molecule can be treated via the equation-of-motion (EOM) ansatz starting from the corresponding molecular wavefunction.
A quantum chemical description is thus possible, via the following two-step procedure consisting of
a) a CC calculation for the molecular system in the reference state
b) a equation-of-motion treatment with removal of one electron or with addition of one electron
The corresponding calculations are invoked by
(a) specifying the quantum chemical approach for the molecule in the reference state, i.e.,
and (b) by requesting the corresponding EOM treatment via
EXCITE=EOMIP (removal of one electron) or EXCITE=EOMEA (addition of one electron).
Furthermore, information concerning from which orbital the electron is removed or to which orbital the electron should be added is needed. This can be accomplished via
the ESTATE_SYM keyword
the %excitations* directive in the lower part of the ZMAT file.
Currently, the EOMIP- and EOMEA-treatments are only possible within the CC framework (available models CC2, CCSD, and CC3) and therein equivalent to the corresponding CC linear-response treatment.
At both the EOMIP-CCSD and the EOMEA-CCSD level, geometry optimizations can be carried out using analytic gradients
EOM-CC calculations for ionized states