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The calculation of excitation energies is initiated by the
data group `$excitations` in which at least the symmetries
(irreducible representations) and the number of the excited states
must be given (for other options see Section 15.2.13).
With the following input the `ricc2` program will calculate the
lowest two roots (states) for the symmetries *A*_{1} and *B*_{1}
of singlet multiplicity
^{}at the CIS, CIS(D) and CC2 level with default convergence thresholds.
Ground-state calculations will be carried
out for MP2 (needed for the CIS(D) model and used as start guess for CC2)
and CC2.
$ricc2
cis
cis(d)
cc2
$excitations
irrep=a1 nexc=2
irrep=b1 nexc=2

The single-substitution parts of the right eigenvectors are stored in
files named `CCRE0-`

*s*`--`

*m*`-`

*xxx*,
where *s* is the number of the symmetry class (irreducible
representation), *m* is the multiplicity, and *xxx* the
number of the excitation within the symmetry class. For the left
eigenvectors the single-substitution parts are stored in files named
`CCLE0-`

*s*`--`

*m*`-`

*xxx*. These files
can be kept for later restarts.

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