  Recently Storm and Nelson [1] (Phys.Rev. E67, 51906 (2003)) introduced the
discrete persistent chain model which contains both features of the freely
jointed chain (FJC) and the wormlike chain (WLC) models. Equation (20) of their
paper is correct only in a special case of large l, the ratio of the
persistence length to the monomer length. This special case is unnecessary
because the general case can be studied just as easily. Working out the general
case, we obtain the force extension relation correct for all values of the
parameter l. This force extension relation reduces to the FJC result at small l
and to the WLC at large l. At small force, it reduces to the result of Rosa et
al (cond-mat/0307015).
