GREENS-FUNCTION METHOD WITH ENERGY-INDEPENDENT VERTEX FUNCTIONS

In conventional Green's function methods the vertex function Gamma is generally energy dependent. However, a model-space Green's function me thod where the vertex function is manifestly energy independent can be formulated using energy-independent effective interaction theories ba sed on folded diagrams and/or similarity transformations. This is disc ussed in general and then illustrated for a 1p1h model-space Green's f unction applied to a solvable Lipkin many-fermion model. The poles of the conventional Green's function are obtained by solving a self-consi stent Dyson equation and model space calculations may lead to unphysic al poles. For the energy-independent model-space Green's function only the physical poles of the model problem are reproduced and are in sat isfactory agreement with the exact excitation energies.