Taking into consideration the overlapping effect of electron clouds between two bonded atoms in a molecule, a modificed potential method is presented which can be used to accurately calculate various cross sections for electron scattering by molecules at intermediate and high energies. The modified complex optical potential is directly employed to calculate the absolute differential, elastic integral and moment transfer cross sections for electron scattering by SO2 in the energy range 100—1000 eV by using the additivity rule at Hartree-Fork level. The quantitative results are compared with those obtained by measurement and other theories wherever available. It is shown that the additivity rule model together with the modified complex optical potential can give the encouraging results, which are much closer to the measurements than the unmodified ones.