Examples of various phenomena which can be studied are:

**Photoexcitation**- If the purpose of a calculation is to predict the energy of photoexcitation,
then the ground-state should first be optimized. Once this is done, then a
C.I. calculation can be carried out using
`1SCF`. With the appropriate keywords (`MECI C.I.=`etc.), the energy of photoexcitation to the various states can be predicted.*n*A more expensive, but more rigorous, calculation would be to optimize the geometry using all the C.I. keywords. This is unlikely to change the results significantly, however.

**Fluorescence**- If the excited state has a sufficiently long lifetime, so that the geometry
can relax, then if the system returns to the ground state by emission of
a photon, the energy of the emitted photon will be less (it will be red-shifted) than
that of the exciting photon. To do such a calculation, proceed as follows:
- Optimize the ground-state geometry using all the keywords for the
later steps, but specify the ground state, e.g.
`MECI``C.I.=3 GNORM=0.01`. - Optimize the excited state, e.g.
`C.I.=3 ROOT=2 GNORM=0.01 MECI`. - Calculate the Franck-Condon excitation energy, using the results of the ground-state calculation only.
- Calculate the Franck-Condon emission energy, using the results of the excited state calculation only.
- If indirect emission energies are wanted, these can be obtained from
the
ΔH
_{f}of the optimized excited and optimized ground-state calculations.

_{0}, then the fluorescing state would be S_{1}. - Optimize the ground-state geometry using all the keywords for the
later steps, but specify the ground state, e.g.
**Phosphorescence**- If the photoemission probability is very low, then the lifetime of the excited
state can be very long (sometimes minutes). Such states can become populated
by S
T
_{1}intersystem crossing. Of course, the geometry of the system will relax before the photoemission occurs. **Indirect emission**- If the system relaxes from the excited electronic, ground vibrational state to
the ground electronic, ground vibrational state, then a more complicated
calculation is called for. The steps of such a calculation are:
- Optimize the geometry of the excited state.
- Using the same keywords, except that the ground state is specified, optimize the geometry of the ground state.
- Take the difference in
ΔH
_{f}of the optimized excited and optimized ground-state calculations. - Convert this difference into the appropriate units.

**Excimers**- An excimer is a pair of molecules, one of which is in an electronic excited state. Such systems are usually stabilized relative to the isolated systems. Optimization of the geometries of such systems is difficult. Suggestions on how to improve this type of calculation would be appreciated.