  Numerical simulation codes are basic tools for designing Free Electron Lasers
(FELs). This paper describes a numerical method for the time-dependent,
three-dimensional simulation of the free electron laser (FEL) using a
rectangular waveguide within overmoded configuration when the radiation
wavelength is much shorter than the waveguide cut-off wavelength. Instead of
developing a new code, the GENESIS simulation code is modified for our purpose.
This method presented here can be used for extending the capacity of GENESIS to
cover this special configuration. The major modification is to apply the metal
boundary conditions on the field equations in a limited rectangular region and
the full Cartesian mesh using the Alternating Direction Implicit (ADI)
integration scheme to solve the field equation remains adopted.
