Phenomenological approximations to the self-energy operator by a generalized X alpha method

The generalized X alpha method, based on a local, energy-independent operat or derived from the exchange-correlation potential of the local-density app roximation by changing the weight of the exchange term, is revisited with t he aim of building an optimized starting point for band-structure calculati ons within the GW method. We find that the optimal choice coincides with th e unmodified local-density approximation potential, i.e., with alpha = 2/3. Moreover, we show that the use of an X alpha method to mimic the self-ener gy effects in the calculation of absorption spectra leads to worse results than the simpler "scissors operator" approach.