Theoretical investigation of the electronic spectrum of pyrazine

The description of Rydberg states by the complete active space self-consistent field (CASSCF) electronic structure method is known to be a difficult topic. In particular, two problems are frequently encountered: (a) the simultaneous presence of valence and Rydberg excited states in the same energy region can potentially lead to artificial valence-Rydberg mixing in the electronic wave functions. (b) Rydberg states have a tendency to be difficult to converge. We have implemented an approach for the consistent description of both valence and Rydberg excited states within the CASSCF electronic structure model. By employing the multiconfigurational second- and third-order perturbation theory (CASPT2/3) methods based on CASSCF reference wave functions, the procedure is verified by comparison with spectroscopic results for the example molecule pyrazine. Vertical excitation energies and other properties have been calculated for various electronic states. Basis sets and active spaces were selected to provide accurate results. Two combinations of aug-cc-pVTZ level basis sets complemented by Rydberg functions have been employed to calculate estimates for the properties of 19 singlet excited states of pyrazine. While many of the assignments made in previous studies could be confirmed, there are also several new aspects emerging from the present investigation.