A NON-DYSON 3RD-ORDER APPROXIMATION SCHEME FOR THE ELECTRON PROPAGATOR

An efficient third-order propagator method to compute ionization poten tials and electron affinities of atoms and molecules is presented. The development is based on the algebraic diagrammatic construction (ADC) representing a specific reformulation of the diagrammatic perturbatio n series of the electron propagator G(omega). In contrast with previou s approximation schemes, relying on the Dyson equation and approximati ons for the self-energy part, the ADC procedure here is applied direct ly to the (N-/+1)-electron parts G(-)(omega) and G(+)(omega), respecti vely, of the electron propagator. This leads to decoupled secular equa tions for the ionization energies ((N-1)-electron part) and electron a ffinities ((N+1)-electron part), respectively. In comparison with the Dyson-type approach, there is a substantial reduction of the secular m atrix dimension opposed by a small additional expense in computing som e second- and third-order contributions to the secular matrix elements . The relationship of the non-Dyson ADC(3) method to coupled cluster m ethods is outlined. (C) 1998 American Institute of Physics.