The rupture properties of a polyethylene knot are studied by first-principles molecular dynamics calculations. While earlier works were conducted in isolated- molecule approximation, we presently report a study focused on a more realistic system where the bulk environment is taken into account through the inclusion of nearest-neighboring chains and a proper set of constraints. Our work shows that the nature of the break in real samples containing knots is essentially intramolecular and that the multimolecules model reproduces results very similar to those of both bulk polyethylene and single-chain systems, thus confirming the relevance of earlier findings.