Second-order Perturbation Theory with Complete and Restricted Active Space Reference Functions Applied to Oligomeric Unsaturated Hydrocarbons

Moughal Shahi, A. R.; Cramer, C. J.; Gagliardi, L.

* Phys. Chem. Chem. Phys.*
**2009**, *11*, 10964
(doi:10.1039/b912607d).

Multiconfigurational second-order perturbation theory based on either a
complete active space reference wave function (CASSCF/CASPT2) or a
restricted active space reference wave function (RASSCF/RASPT2) has been
applied to compute one-electron ionization potentials and vertical
electronic energy differences of oligomers of length *n* formed from
ethylene
(*n* = 1-10), acetylene (*n* = 1-5), and phenylene
(*n* = 1-3) subunits. The
RASSCF/RASPT2 approach offers an accuracy similar to CASSCF/CASPT2 at
significantly reduced computational expense (both methods show good
agreement with experimental data where available). It is shown that RASPT2
increases the applicability of CASPT2-type approaches by permitting the use
of larger active spaces.