The MolSSI Driver Interface (MDI) Project provides a standardized API for fast, on-the-fly communication between computational molecular sciences codes. This greatly simplifies the process of implementing methods that require the cooperation of multiple software packages and enables developers to write a single implementation that works across many different codes. The API is sufficiently general to support a wide variety of techniques, including QM/MM, ab initio MD, machine learning, advanced sampling, and path integral MD, while also being straightforwardly extensible. Communication between codes is handled by the MDI Library, which enables tight coupling between executables using either the MPI or TCP/IP methods.