Calculation of hydrodynamic properties of macromolecular bead models with overlapping spheres

For the calculation of hydrodynamic properties of rigid macromolecules usin g bead modelling, models with overlapping beads of different sizes are used in some applications. The hydrodynamic interaction tensor between unequal overlapping beads is unknown, and an oversimplified treatment with the Osee n tensor may introduce important errors. Here we discuss some aspects of th e overlapping problem, and explore an ad hoc form of the interaction tensor , proposed by Zipper and Durchschlag. We carry out a systematic numerical s tudy of the hydrodynamic properties of a two-spheres model, showing how the Zipper-Durchschlag correction removes efficiently the numerical instabilit ies, and predicts the correct limits.