Improved calculation of rotational diffusion and intrinsic viscosity of bead models for macromolecules and nanoparticles.

The conventional Kirkwood-Riseman calculation of the hydrodynamic properties of bead models gives abnormal results for rotational quantities and the intrinsic viscosities for models with a few beads or when one bead is dominant. The reason is that beads are treated as point sources of friction. This can be remedied by introducing terms that are neglected in the conventional treatment of orders 0 and -3 in interbead distances. An alternative strategy is the cubic substitution in which each bead is replaced by a cubic array of minibeads. These procedures require a computational overload that, in the case of the intrinsic viscosity, can be avoided using an estimate of the correction due to the nonzero volume of the beads. We have found how such a correction can be estimated from the geometry of the model, and its application yields results that are within the range of typical experimental errors.