Page 66 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

A COUPLED MCSCF-PERTURBATION TREATMENT OF ELECTRONIC SPECTRA 51

The comparison to the results obtained using the SCI/MP2 approach [45,68] leads to
unquestionable conclusions: not only the SCI/MP2 method does not provide acceptable
transition energies for the lowest valence and Rydberg states but it misses some of them
and does not provide any good energetical ordering of the excited states. Even if this
method presents interesting computational advantages, it can only provide a flimsy
quantitative electronic spectrum, as anticipated in section 3.1 and outlined in reference [22].

5. Conclusions and prospects
The present approach is one of the second-generation multireference perturbation treatments
first opened by the CIPSI algorithm 20 years ago. Even if the spirit of these new treatments
is different, mainly because the reference space is chosen on its completeness rather than on
energetical criteria, it remains that the unavoidable problems of disk storage, bottleneck of
variational approaches, can now be conveniently transferred to the problem of CPU time
which is less restrictive.
The methodology presented here expands the recent CASPT2 approach to more flexible
zeroth-order variational spaces for a multireference perturbation, either in the Moller-Plesset
scheme or in Epsein-Nesbet approach [70-72]. Furthermore, it allows for the use of a wide
set of possible correlated orbitals. These two last points were discussed elsewhere [34].
The reliability of this method for the evaluation of (vertical) electronic spectra has been
clearly established in the present work, and further calculations on other molecules
(ethylene, vinylydene… for example) have confirmed the very promising potentialities of
such an approach that avoids the possible artefacts brought in by any arbitrary truncated
CIs when dealing with excited states [49]. We also emphasize that this methodology is able
to give reliable splittings between states ranging from 10 kcal/mol to more than 10 eV.

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