Many-body Green's-function calculations on the electronic excited states of extended systems

Electron correlation corrections to the excitation energy of the lowest-lyi ng singlet exciton state of polyethylene are evaluated with the aid of the quasiparticle energies obtained from second-order many-body perturbation th eory and from the second-order inverse Dyson equation. A simple approximati on is proposed to avoid the evaluation of the quasiparticle energies for hi gh- and low-lying energy bands, which is particularly problematic in extend ed-system calculations. The inclusion of both the electron correlation effe cts and diffuse basis functions is important for the proper description of the exciton state. The electron correlation corrections calculated by this method appear to be too large, probably due to the neglect of the screening effects of the quasiparticle interactions. (C) 2000 American Institute of Physics. [S0021-9606(00)31017-0].