The decomposition of trimethylene embedded in an argon environment is studied using classical trajectory simulations. The trimethylene intramolecular forces are calculated directly from semiempirical electronic structure theory at each trajectory step, and the intermolecular forces are determined from Lennard-Jones 12-6 potential energy functions. When the argon is in a low to moderate density fluid phase, it does not affect the rate of trimethylene decomposition or product branching ratios, but at high densities, the H-transfer reaction leading to propene is favored over cyclization.