Electronic excitation and ionization spectra of azabenzenes: Pyridine revisited by the symmetry-adapted cluster configuration interaction method

Electronic excited and ionized states of pyridine were reinvestigated by the symmetry-adapted cluster configuration interaction ~SAC-CI! method using an extended basis set and a wide active space. The present SAC-CI results for the singlet and triplet excited states are greatly improved and agree well with the experimental observations, providing a firm assignment of all low-lying n→p* and p→p* valence excited states observed in the vacuum ultraviolet spectrum and electron energy-loss spectrum. The ionization potentials were reexamined by the SAC-CI general-R ~R represents excitation operator! method. The first four ionization potentials are greatly improved compared with our previous results obtained by the SAC-CI singleand double-R ~SD-R! method. The present theoretical ionization potentials are in good agreement with the experimental values in high-resolution synchrotron photoelectron spectrum for energy regions up to 25 eV ~which contain outerand inner-valence regions!, and give a detailed theoretical assignment for the photoelectron spectra. © 2001 American Institute of Physics. @DOI: 10.1063/1.1351880#