  The non-resonant tunneling regime for charge transfer across nanojunctions is
critically dependent on the so-called \beta{} parameter, governing the
exponential decay of the current as the length of the junction increases. For
periodic materials, this parameter can be theoretically evaluated by computing
the complex band structure (CBS) -- or evanescent states -- of the material
forming the tunneling junction. In this work we present the calculation of the
CBS for organic polymers using a variety of computational schemes, including
standard local, semilocal, and hybrid-exchange density functionals, and
many-body perturbation theory within the GW approximation. We compare the
description of localization and \beta{} parameters among the adopted methods
and with experimental data. We show that local and semilocal density
functionals systematically underestimate the \beta{} parameter, while
hybrid-exchange schemes partially correct for this discrepancy, resulting in a
much better agreement with GW calculations and experiments. Self-consistency
effects and self-energy representation issues of the GW corrections are
discussed together with the use of Wannier functions to interpolate the
electronic band-structure.
