The electronic structure of nitrobenzene
(C6H5NO2) has been studied by means of the complete active
space self-consistent field (CASSCF) and multi-state second-order
perturbation (MS-CASPT2) methods. To this end, an active space
of 20 electrons distributed in 17 orbitals has been selected to
construct the reference wave function. In this work, we have
calculated the vertical excitation energies and the energy barrier for
the dissociation of the molecule on the ground state into phenyl
and nitrogen dioxide. After applying the corresponding vibrational
corrections to the electronic energies, it is demonstrated that the MS-CASPT2//CASSCF values obtained in this work yield an excellent agreement between calculated and experimental data. In addition, other active spaces of lower size have been applied to the system in order to check the active space dependence in the results.
