So far, the demonstrator MD-Flexible for the AutoPas particle simulation library was limited to a single process. MD-Flexible now has been turned into a massively parallel application using MPI for inter-processor communication and adaptive domain decomposition for workload balancing. It subdivides the simulation domain into a regular grid which is either balanced by ALL's Tensor method or by the here presented Inverted Pressure method. After taking a brief introduction into Molecular Dynamics and Adaptive Domain Decomposition, we take a look at the technologies involved in the parallelization of MD-Flexible and compare it to other well known massively parallel Molecular Dynamics applications. Before going into detail about the implementation of massive parallelism, we explain on a high level how MD-Flexible worked before and what has changed during the implementation. Next, we describe the details about the adaptive domain decomposition, the resulting inter-process communication and other requirements that arise from the parallelization. The result has been evaluated by running four scenarios with up to 128 processes.