Correlation effects in the valence ionization spectra of large conjugated molecules: p-Benzoquinone, anthracenequinone and pentacenequinone

A review of an extensive series of theoretical studies of the valence one-electron and shake-up ionization spectra of polycyclic aromatic hydrocarbons is presented, along with new results for three planar quinone derivatives, obtained using one-particle Green's function (1p-GF) theory along with the so-called third-order algebraic diagrammatic construction [ADC(3)] scheme and the outer-valence Green's function (OVGF) approximation. These results confirm both for the pi- and sigma-band systems the rapid spreading, upon increasing system size, of many shake-up lines with significant intensities at outer-valence energies. Linear regressions demonstrate that with large conjugated molecules the location of the shake-up onset in the pi-band system is merely determined by the energy of the frontier (HOMO, LUMO) orbitals. Electron pair removal effects are found to almost compensate the electron relaxation effects induced by ionization of pi-levels, whereas the latter effects strongly dominate the ionization of more localized lone-pair (n) levels, and may lead to inversions of the energy order of Hartree-Fock (HF) orbitals. Therefore, although it increases upon a lowering of the HF band gap, and thus upon an increase of system size, the dependence of the one-electron ionization energies onto the quality of the basis set is lesser for pi-levels than for sigma-levels relating to electron lone pairs (n). Basis sets of triple- and quadruple-zeta quality are therefore required for treatments of the outermost pi- and n-ionization energies approaching chemical accuracy [1 kcal/mol, i.e. 0.04 eV]. When 1p-GF theory invalidates Koopmans' theorem and the energy order of HF orbitals, a comparison with Kohn-Sham orbital energies confirms the validity of the meta-Koopmans' theorem for density functional theory. (C) 2009 Elsevier B.V. All rights reserved.