`$scfinstab rpas`-

for closed-shell singlet excitations, `$scfinstab rpat`-

for closed-shell triplet excitations, and `$scfinstab urpa`-

for excitations out of spin-unrestricted reference states.

`$scfinstab ciss`-

for closed-shell singlet excitations, `$scfinstab cist`-

for closed-shell triplet excitations, and `$scfinstab ucis`-

for excitations out of spin-unrestricted reference states.

Next, the IRREPs of the excitations need to be defined, which is again
accomplished using `$soes`

. For example, to calculate the 17
lowest excitations in IRREP b1g, the 23 lowest excitations in IRREP
eu, and all excitations in IRREP t2g, use

$soes b1g 17 eu 23 t2g alland run

Note that `$soes` specifies the IRREP of the *excitation
vector* which is not necessarily identical to the IRREP of the
*excited state(s)* involved. In general, the IRREP(s) of the
excitation(s) from the ground to an excited state is given by the
direct product of the IRREPs of the tow states. For example, to
calculate the first *A*_{2} state in a *C*_{2v}-symmetric molecule with
a *B*_{2} (open-shell) ground state, it is necessary to specify

$soes b1 1

The number of excitations that have to be calculated in order to cover a
certain spectral range is often difficult to determine in advance. The
total number of excitations within each IRREP as provided by the
`define` `ex`

menu may give some hint. A good strategy is to start
with a smaller number of excitations and, if necessary, perform a
second `escf` run on a larger number of states using the already
converged excitation vectors as input.

To compute absorption and CD spectra, it is often sufficient to
include optically allowed transitions only. This leads to substantial
reduction of computational effort for molecules with higher
symmetry. For example, in the UV-VIS spectrum of an *O*_{h} symmetric
molecule, only *t*_{1u} excitations are optically allowed. The IRREPs
of the electric and magnetic dipole moments as well as of the electric
quadrupole moment are displayed automatically in the `define` `ex`

menu.

If a large number of states is to be calculated, it is highly recommended to provide extra memory by specifying

`$rpacor`*m*

By specifying

`$spectrum`*unit*- and/or
`$cdspectrum`*unit*

`spectrum`

and/or `cdspectrum`

. As above, unit specifies the
energy unit and may be `ev`

, `nm`

, `1/cm`

, or
`a.u.`

(default). The files `spectrum`

and `cdspectrum`

may
conveniently be used for further processing, e.g., using a plotting
program such as Gnuplot.