  We introduce a multimodel approach to solve coupled cluster equations,
employing a quasi Newton algorithm for the ground state and an Olsen algorithm
for the excited states. In these algorithms, both of which can be viewed as
Newton algorithms, the Jacobian matrix of a lower level coupled cluster model
is used in Newton equations associated with the target model. Improvements in
convergence then implies savings for sufficiently large molecular systems,
since the computational cost of macroiterations scales more steeply with system
size than the cost of microiterations. The multimodel approach is suitable when
there is a lower level Jacobian matrix that is much more accurate than the
zeroth order approximation. Applying the approach to the CC3 equations, using
the CCSD approximation of the Jacobian, we show that the time spent to
determine the ground and valence excited states can be significantly reduced.
We also find improved convergence for core excited states, indicating that
similar savings will be obtained with an explicit implementation of the
core-valence separated CCSD Jacobian transformation.
