Modelling streaming potential generation in partially saturated porous media is of high interest for hydrological and reservoir studies as it allows to relate water fluxes to measurable electrical potential distributions. One key parameter to describe the electrokinetic coupling is the effective excess charge density. This document describes the Matlab function fQeff_VG to determine the effective excess charge density as a function of the saturation using the model described in Jougnot et al. (2012). This version of the code is based on the van Genuchten hydrodynamic parameters.