SYSTEMATIC VERTEX CORRECTIONS THROUGH ITERATIVE SOLUTION OF HEDINS EQUATIONS BEYOND THE GW APPROXIMATION

We present a general procedure for obtaining progressively more accura te functional expressions for the electron self-energy by iterative so lution of Hedin's coupled equations. The iterative process starting fr om Hartree theory, which gives rise to the GW approximation, is contin ued further, and an explicit formula for the vertex function from the second full cycle is given. Calculated excitation energies for a Hubba rd Hamiltonian demonstrate the convergence of the iterative process an d provide further strong justification for the GW approximation. [S003 1-9007(98)05360-5].