An ab initio quantum chemical study of vertically excited singlet states of pyrimidine

Ab initio calculations of a number of singlet electronic states of pyrimidine are presented. In addition to the ground ¹A₁ state, ten valence excited states were studied: three ¹A₁ and three ¹B₂ states, which are ππ* excited states, and two ¹B₁ and two ¹A₂ states, which are nπ* excited. Rydberg states were encountered, but since these were irrelevant to the present study, the basis set employed was not designed to describe these states well, and their energy relative to the valence states will be incorrect. Such spurious states may perturb the valence states by accidental near-degeneracy, and care was taken to effectively remove them to prevent such perturbation. The calculation were performed by the CASSCF and MRCI methods, using a modest basis set (C, N: 3s3p1d; H: 2s; 116 basis functions) of ANO type. Energies, dipole and quadrupole moments, transition moments, and oscillator strengths are reported. All calculations were made using the experimental equilibrium geometry, and all excitation data are therefore vertical. The MRCI excitation energies, corrected for unlinked clusters, give results in good agreement with known experimental data. The error in the computed excitation energy is in no case larger than 0.4 eV. In some cases, alternative assignments of experimental energies are indicated.