Transforming nonlocality into a frequency dependence: A shortcut to spectroscopy

Measurable spectra are often derived from contractions of many-body Green\textquoterights functions. One calculates hence more information than needed. Here we present and illustrate an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, a dynamical but local and real potential yields the spectral function needed to describe photoemission. We discuss for model solids the frequency dependence of this \textquoteleft\textquoteleftphotoemission potential\textquoteright\textquoteright stemming from the nonlocality of the corresponding self-energy.