  The mechanical resistance of a polyethylene strand subject to tension and the
way its properties are affected by the presence of a knot is studied using
first-principles molecular dynamics calculations. The distribution of strain
energy for the knotted chains has a well-defined shape that is very different
from the one found in the linear case. The presence of a knot significantly
weakens the chain in which it is tied. Chain rupture invariably occurs just
outside the entrance to the knot, as is the case for a macroscopic rope.
