Knowledge Bank

New correlated ab initio vertical electronic spectra are reported for the lowest energy valence multiplet $(1\pi g32\pi u)$ and mixed character states $(1\pi g35\sigma g\to 1,3\Pi g)$ of $CO2$. Multiconfiguration self-consistent field (MCSCF) theory is used to define a set of orbitals for each state in a near Hartree-Fock limit basis set. Configuration interaction (CI) calculations are carried out which include single excitations into the virtual MCSCF space. Natural orbitals of the CI wavefunctions are used to carry out a second set of CI calculations. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi g$ mixed character state is found to lie 0.3 to 0,5 eV below the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta u$ valence state in all of the correlated ab initio results. The calculated spectrum will be compared with other theoretical spectra and with experimental results.