A test of the method of images at the surface of molecular materials

The method of images is tested by comparing two ways of calculating the polarization energy in crystalline fullerene C60 and in bulk amorphous polyethylene (PE): (i) treating the whole molecular material microscopically, and (ii) replacing part of the material by a uniform dielectric continuum of the same relative permittivity. The method of images is accurate to within 5% once the distance of the charge from the surface of the dielectric continuum exceeds about twice the average spacing between the polarizable units in the molecular material. For C60 crystals the method of images always overestimates the magnitude of the polarization energy, partly because it ignores the reduction in the relative permittivity of the dielectric continuum near its surface. For amorphous PE the method of images can overestimate or underestimate the true result, depending on the local density around the charge.