Electronic excitation energies of pyrimidine studied using coupled cluster response theory
Coupled cluster (CC) response theory is used to study the excited singlet states of pyrimidine. Both valence excitations (π → π* and π → π*) and excitations to the lowest n = 3, 4 Rydberg states are calculated. The dependency of vertical excitation energies on several factors are investigated, such as (i) the effect of different molecular geometries on the vertical excitation energies; (ii) the dependency of the predictions on various features of the basis set (diffuseness, cardinal number in the correlation consistent series, inclusion of molecule centered functions and the position of such); and (iii) the level of electron correlation model (studied by a hierarchy of CC methods: CCS, CC2, CCSD and triple corrected CCSD by means of CCSDR(3)). Geometrical relaxation and zero-point vibrational motion in the excited states are accounted for by explicit calculations for the valence states and indirectly through studying ionized states for the Rydberg states. Best estimates of vertical and 0-0 transition energies are given and compared to previous calculations and experiments. Excellent agreement with available experimental 0-0 energies is obtained. For the lowest excited state we question a structure recently derived from experimental data, and argue that reasonable agreement with experiment is obtained with our structure.