The complete cohesion of two thin films composed of melts of C99H200 has been achieved by simulation. The simulation employs a new ＇mapping/reverse mapping＇ technique that is orders of magnitude more efficient than conventional molecular dynamics simulations of fully atomistic models. Three distinct processes, each with its own time scale, can be resolved. From fastest to slowest, these are: 1. the healing of the density profile between the two films, 2. the redistribution of chain ends, and 3. the complete intermixing of the segments from the original two films. This simulation has the capability of producing new insights into the mechanism of interdiffusion across an interface and the mechanism of the development of cohesive strength at the interface.