  The process of force generation by the growth of biopolymers is simulated via
a Langevin-dynamics approach. The interaction forces are taken to have simple
forms that favor the growth of straight fibers from solution. The
force-velocity relation is obtained from the simulations for two versions of
the monomer-monomer force field. It is found that the growth rate drops off
more rapidly with applied force than expected from the simplest theories based
on thermal motion of the obstacle. The discrepancies amount to a factor of
three or more when the applied force exceeds 2.5kT/a, where a is the step size
for the polymer growth. These results are explained on the basis of restricted
diffusion of monomers near the fiber tip. It is also found that the mobility of
the obstacle has little effect on the growth rate, over a broad range.
