  As simulation system, the variety of physics processes implemented is one of
the most important functionalities. In that sense, Geant4 is one of the most
powerful simulation toolkits. Its flexibility and expansibility brought by
object-oriented approach make it possible for us to easily assimilate external
simulation packages into the Geant4 system as modules of physics processes. We
developed an interface for using EGS4, which is another of the most well-known
simulation package for electromagnetic physics, in Geant4. By means of this
interface, EGS4 users can share Geant4 powerful resources, such as geometry,
tracking, etc. It is also important that it can provide a common environment
for comparison tests between EGS4 and Geant4. In this paper, we describe our
design and implementation of the interface.

Computational Physics