Simplified Ring and Ladder Renormalizations in Electron-Propagator Calculations of Molecular Ionization Energies

The self-energy operator of the <i>ab initio</i> Dyson quasiparticle equation generates orbital-relaxation and differential-correlation corrections to Koopmans predictions of electron binding energies. Among the most important corrections are terms that may be expressed as ring and ladder diagrams. Inclusions of such terms in all orders of the fluctuation potential constitute renormalizations. The ability of several renormalized self-energies to predict molecular ionization energies has been tested versus reliable computational and experimental standards. These results reveal the superior accuracy and efficiency of several new-generation electron-propagator methods. They also demonstrate the strengths and weaknesses of self-energies that include ring or ladder renormalizations only and of methods that allow interactions between these terms. Whereas a simplified ladder method produces useful results, its simplified ring counterpart is more computationally efficient, but less accurate. New-generation alternatives to both methods are more accurate and efficient. No adjustable parameters are included in the generation of reference orbitals or in the formulation of the self-energy approximations examined in this work.