Theoretical study of the singlet electronically excited states of N4

Vertical excitation energies for the lowest eleven singlet states of Td N4 were calculated using the TD-DFT method with the B3LYP functional, and at the EOM-CCSD level of theory. The vertical excitation energies for the five lowest-lying excited states were also obtained using the state-averaged CASSCF, CASPT2, CASPT3, and MRCI + Q methods. Our results show that the five lowest-lying states are of valence character. EOM-CCSD/d-aug-cc-pVTZ calculations predict that there are two weakly allowed optical transitions of T2 symmetry at 10.44 and 10.82 eV. The transition to the third T2 state, which is predicted to be at 10.89 eV, has an oscillator strength about one order of magnitude higher.