The independent atom model representation (IAM) in combination with the additivity rule (AR) is customary accepted as a good approximation to describe electron and positron scattering processes with polyatomic molecules at intermediate and high energies (typically above 100 eV). Within this approach, interference terms are not properly evaluated and are usually ignored in the cross section calculations. However, through a numerical solution of the Lippmann-Schwinger equation for three dimensional multicentre potentials we here demonstrate how interference terms can affect to the calculated differential and integral cross section values even at such relatively high energies. Practical corrections to the AR approach for both differential and integral electron and positron scattering cross section calculations from molecules are also proposed. (c) 2015 Elsevier B.V. All rights reserved.