4. Conclusion

The ICCE method can be seen as a general diagonalization method for Hermitian or symmetrical operators acting on a space admitting a tensorial structure. It is particularly well suited to molecular spectroscopy, for which it was developed. It is not the only method used in this field (an exhaustive catalog would be beyond the scope of this article), but its flexibility makes it applicable to a wide range of contexts.

The computational cost of an ICCE step is that of a matrix diagonalization and, if the step in question is not the last, or if matrix representations of observables have to be calculated, the cost of transformations from base change to the basis of the eigenvectors obtained must be added. There are as many transformations to perform as there are distinct types of terms in the Hamiltonian and/or observables. This number can be significant, depending...