Using multiple engines in the Virtual Monte Carlo package

The Virtual Monte Carlo (VMC) package together with its concrete implementations provides a unified interface to different detector simulation transport engines such as GEANT3 or GEANT4. However, so far the simulation of one event was restricted to the usage of one chosen engine. We introduce here the possibility to mix multiple engines within the simulation of one event. Depending on user conditions the simulation is then effectively split among the chosen engines. Among others, these conditions can depend on phase space, geometry, particle type or an arbitrary combination. This development hence offers the possibility to choose multiple engines for an event simulation profiting from each of their advantages or specific capabilities under different conditions. Furthermore, it allows for the implementation of fast simulation kernels on VMC level which can then easily be dispatched to during simulation. This also makes stand-alone fast simulation possible since such a fast simulation kernel in essence acts as a fully functional engine. This development opens many new opportunities for the usage of the VMC package and its functionality. It allows for extended studies of detector simulation engines, their interplay and, especially concerning customized user implementations of fast simulations, it provides vital new flexibility.