  In extracting deformation parameters from multipole moments for deformed
nuclei, one commonly uses the formulas which are based on a sharp-cut density
distribution. We discuss a possible ambiguity for this procedure and clarify
the role of diffuseness parameter of the density distribution. For this
purpose, we use a deformed Woods-Saxon density as well as a density
distribution obtained from the self-consistent relativistic mean-field (RMF)
model. We show that the formula using a root-mean-square radius instead of a
sharp-cut radius requires a large correction due to a finite surface
diffuseness parameter even for stable nuclei. An implication to neutron-rich
nuclei is also discussed.
