Guidelines for Using the Iterative Solver

The iterative solver works most efficiently when it is enabled for designs that do not contain many excitations. (For example, the number of excitations is less than twice the number of processors.)
If you choose to take advantage of the iterative solver, and your analysis includes interpolating sweeps or discrete sweeps, the adaptive solution should be well converged at the higher end of the frequency band.
The Relative Residual provides a stopping criteria. The residual measures the convergence of the iterative solver to the solution of the matrix equation. Its value affects the performance of the iterative solver as follows:

Default is 1E-6. This gives accurate S-parameters and fields, indistinguishable from those generated by the direct solver. Ansys recommends this residual.
With a larger residual, for example, 1E-3 or 1E-2, the iterative process will stop with fewer iterations and the solution will be less converged. S-parameters won't differ much from those of a direct solution, for example, a difference in third or second digit. Fields and antenna patterns are visually the same.
A residual of 0.1 can be used for quick adaptive mesh refinement early in the adaptive process, but S-parameters will be noticeably different.
A residual of 1 should never be used. The interface will not allow a residual above 0.1.