Running excitation energy calculations:

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 A₁ and B₁ 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.