Theoretical study of the singlet electronically excited states of N-4

Vertical excitation energies for the lowest eleven singlet states of T-d N- 4 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 fiv e lowest-lying excited states were also obtained using the state-averaged C ASSCF, 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 calcu lations predict that there are two weakly allowed optical transitions of T- 2 symmetry at 10.44 and 10.82 eV. The transition to the third T-2 state, wh ich is predicted to be at 10.89 eV, has an oscillator strength about one or der of magnitude higher. (C) 2001 Elsevier Science B.V. All rights reserved .