- solution of the CCS/CIS eigenvalue problem to generate
reasonable start vectors; the eigenvectors are converged in
this step only to a remaining residual norm
`preopt`

- pre-optimization of the eigenvectors by a robust
modified Davidson algorithm (see ref. [10])
using the
`LINEAR CC RESPONSE SOLVER`until the norm of all residuals are below`preopt`

, combined with a DIIS extrapolation for roots assumed to be converged below the threshold`thrdiis`

. - solution of the nonlinear eigenvalue problem with a DIIS
algorithm using the
`DIIS CC RESPONSE SOLVER`until the norm of the residuals are below the required threshold`conv`

- almost degenerate roots in the same symmetry class
- complex roots (break down of the CC approximation close in the neighbourhood of conical intersections)
- large contributions from double excitations

`thrdiis`

(controlling the DIIS extrapolation in the
linear solver) should be set about one order of magnitude
smaller than the smallest difference between two eigenvalues
and `preopt`

(controlling the switch to the DIIS solver)
again about one order of magnitude smaller then `thrdiis`

.
Tighter thresholds or difficult situations can make it necessary
to increase the limit for the number of iterations `maxiter`

.

In rare cases complex roots might persist even with tight convergence
thresholds. This can happen for CC2 and CIS(D) close
to conical intersections between two states of the same symmetry,
where CC response can fail due to its non-symmetric Jacobian.
In this case one can try to use instead the ADC(2) model.
But the nonlinear partitioned form of the eigenvalue problem used
in the `ricc2` program is not well suited to deal with such situations.

Large contributions from double excitations can not be monitored
in the output of the (quasi-) linear solver.
But it is possible to do in advance a CIS(D) calculation.
The CIS(D) results for the `||T2||`

diagnostic correlate usually
well with the CC2 results for this diagnostic.
Else the DIIS solver will print the `||T2||`

diagnostics
in each iteration if the print level is set .
States with large double excitation contributions converge notoriously
slow (a consequence of the partitioned formulation used in the `ricc2` program).
However, the results obtained with second-order methods for
double excited states will anyway be poor. It is strongly recommended
to use in such situations a higher-level method.