  The effective hydrodynamic radius is usually assumed to be a constant in
testing the Stokes-Einstein relation by its variants. We have performed
molecular dynamics simulations with ortho-terphenyl and Kob-Andersen model to
examine the assumption and found the effective hydrodynamic radius is not a
constant but decreases with decreasing temperature. The variant of
Stokes-Einstein relation is not established but Stokes-Einstein relation is
valid by considering the changes of the effective hydrodynamic radius. We
propose the assumption should be seriously evaluated when using variants to
test Stokes-Einstein relation.
