Ab initio and density functional calculations of the energies of the singlet and triplet valence excited states of pyrazine

A total of 33 ab initio or density functional schemes are applied to evaluate the vertical excitation energies of each of six pyrazine triplet excited states while 22 schemes are applied for each of eight singlet excited states; recent EOM-CCSD(T) results for the singlet states are also considered. The highest quality results are obtained using CASPT2, B3LYP, and EOM-CCSD(T) methodologies. Time-dependent density functional methods are found to produce excitation energies for triplet states in excellent agreement with those evaluated directly. For singlet states, the state-average method, which is commonly used to treat spin contamination, is found to give poor results compared to those given by time-dependent density functional theory. While the notionally most reliable calculations support the (contentious) assignment of B-3(1u) and B-3(2u) as given by Walker and Palmer, the errors associated with the methods are too large to provide an authoritative assignment. Reorganization energy calculations indicate that (3)A(u) is very broad and has been incorrectly assigned; they also suggest that this state could be responsible for the observed chaos in the S-0 --> T-1 absorption spectrum of pyrazine crystal as well as the observed high vibrational relaxation efficiency of T-1.