The electronic spectrum of pyrrole

The electronic spectrum of pyrrole has been investigated by performing calculations using a hierarchy of coupled-cluster models consisting of CCS, CC2, CCSD, and CC3. Basis-set effects have been investigated by carrying out calculations using correlation-consistent basis sets augmented with functions especially designed for the description of Rydberg states. Oscillator strengths, excited state dipole moments, and second moments of the electronic charge distributions have been used to characterize the electronic transitions and final states. Structures and vibrational frequencies have been calculated for a few selected states, and the importance of distinguishing between vertical and adiabatic transitions for accurate comparison with experiment has been emphasized. The experimental spectrum has been scrutinized in the relevant energy region, and the accuracy of recent calculations [CASPT2, MRMP, ADC(2)] has been critically reexamined. (C) 1999 American Institute of Physics. [S0021-9606(99)30326-3].