  The complexity of the Round Damped Detuned Structue (RDDS) for the JLC/NLC
main linac is driven by the considerations of rf efficiency and dipole
wakefield suppression. As a time and cost saving measure for the JLC/NLC, the
dimensions of the 3D RDDS cell are being determined through computer modeling
to within fabrication precision so that no tuning may be needed once the
structures are assembled. The tolerances on the frequency errors for the RDDS
structure are about one MHz for the fundamental mode and a few MHz for the
dipole modes. At the X-band frequency, these correspond to errors of a micron
level on the major cell dimensions. Such a level of resolution requires highly
accurate field solvers and vast amount of computer resources. A parallel
finite-element eigensolver Omega3P was developed at SLAC that runs on massively
parallel computers such as the Cray T3E at NERSC. The code was applied in the
design of the RDDS cell dimensions that are accurate to within fabrication
precision. We will present the numerical approach of using these codes to
determine the RDDS dimensions and compare the numerical predictions with the
cold test measurements on RDDS prototypes that are diamond-turned using these
dimensions.
