The direct simulationMonte Carlo (DSMC), a kinetic method, was recently expanded to include the simulation of homogeneous condensation in the free expansion of water plumes and the results show that the nucleation rate is a key factor to accurately modeling of condensation phenomenon. In this work, we performed MD simulations of a free expansion to explore the microscopic mechanisms of water dimer formation and the bimolecular formation was found to be the main mechanism. The bimolecular dimer formation probabilities through collision between two water molecules for different collision energies were obtained by MD simulations and found to decrease with collision energy. The formation probabilities and post-collisional velocity and energy distributions were then integrated into DSMC simulations of a free expansion of an orifice condensation plume with different chamber stagnation temperatures. The dimer mole fraction increases with distance from the orifice and becomes constant after a distance of about two orifice diameters. The terminal dimer mole fraction was found to decrease with chamber stagnation temperatures but higher than experimentally observed.