The electronic spectrum of pyrrole

The electronic spectrum of pyrrole has been investigated by performing calc ulations using a hierarchy of coupled-cluster models consisting of CCS, CC2 , CCSD, and CC3. Basis-set effects have been investigated by carrying out c alculations using correlation-consistent basis sets augmented with function s especially designed for the description of Rydberg states. Oscillator str engths, excited state dipole moments, and second moments of the electronic charge distributions have been used to characterize the electronic transiti ons and final states. Structures and vibrational frequencies have been calc ulated for a few selected states, and the importance of distinguishing betw een vertical and adiabatic transitions for accurate comparison with experim ent 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].