  A formula is derived for stiffness of a polymer chain in terms of the
distribution function of end-to-end vectors. This relationship is applied to
calculate the stiffness of Gaussian chains (neutral and carrying electric
charges at the ends), chains modeled as self-avoiding random walks, as well as
semi-flexible (worm-like and Dirac) chains. The effects of persistence length
and Bjerrum's length on the chain stiffness are analyzed numerically. An
explicit expression is developed for the radial distribution function of a
chain with the maximum stiffness.
