Three Approximations to the Nonlocal and Energy-Dependent Correlation Potential in Electron Propagator Theory

摘要

The historical dilemma that pits accuracy against interpretability in molecular electronic structure calculations may be resolved with electron propagator theory. In the interpretation of electron binding energies, electron propagator theory provides a systematically improvable description of electron correlation while retaining one-electron concepts, such as Dyson orbitals, of wide-ranging interpretive facility. The nonlocal, energy-dependent correlation potential, known as the self-energy, which occurs in the one-electron equations of electron propagator theory may be approximated in practical calculations. The partial third-order approximation has been employed in many calculations on large molecules. Applications to fragments of nucleic acids show how qualitative, orbital-based insights emerge from calculations that suffice to quantitatively assign photoelectron spectra. A quasiparticle virtual orbital method for improving the efficiency of this method shows considerable promise. Simple perturbative approaches may be combined with renormalizations that incorporate finalstate orbital relaxation effects with the use of reference ensembles that correspond to Slater’s transition state method. Such approximations produce useful predictions for core as well as valence electron binding energies. Another approximation that is based on highly correlated reference states and a renormalized treatment of the self-energy shows considerable flexibility in the accurate prediction of electron binding energies. This method suffices to make definitive assignments of photoelectron spectra of double Rydberg anions, species whose ground state electronic structure requires assignment of electrons to nonvalence orbitals.