Dipolar unit size in coupled dipole calculations of the scattering matrix elements

The coupled-dipole method is widely used to calculate the light-scattering matrix S from arbitrary particles. An important parameter in the model is the size of the dipolar subunits. Usually a size of ~1/10 to ~1/20 of the wavelength of the incident light is sufficient for accurate calculations. However, it was noted that accurate S3 4 calculations require much smaller dipolar subunits. We show that this conclusion is too pessimistic, by examining the sensitivity of the S34 elements on surface roughness of spherical particles. Furthermore we give an example of an accurate S34 calculation with dipolar subunits as large as 1/10 of the wavelength.