The electronic structure of molecules by a many-body approach III. Ionization potentials and one-electron properties of furan and thiophene

The valence ionization potentials (IP's) of furan and thiophene are studied by an ab initio many-body approach which includes the effects of electron correlation and reorganization beyond the Hartree—Fock approximation. For both molecules it is found that the ordering of the IP's as obtained in the Hartree—Fock approximation is correct. The assignment made for furan agrees with the ab initio calculation of Siegbahn, but it does not agree with the ordering proposed by Derrick et al. from their experimental investigations. For thiophene both the ordering of Derrick et al. and the one of Gelius et al. is shown to be incorrect concerning the position of the 1b₁(π) IP. For both molecules the first two IP's are due to the 1a₂(π) and the 2b₁(π) molecular orbitals. For furan four orbitals of σ-type symmetry are placed between the 2b₁ and the 1b₁ π-orbitals, for thiophene there is only one. Several one-electron properties are calculated in the one-particle approximation and compared with experimental and other theoretical data. The localized molecular orbitals are also discussed.