HFSS Integral Equation (IE) Solver
The HFSS integral equation solver (HFSS-IE) is an optional, add-on solver that uses the method of moments (MoM) technique to solve for the sources or currents on the surfaces of conducting and dielectric objects in open regions. HFSS-IE is effective for radiation and scattering studies of large, mostly conducting structures. It is available in the industry-standard HFSS interface to share geometry, material and certain critical solver technology. As with HFSS, adaptive refinement is used to generate the optimum mesh giving the user added confidence in the results. The solver uses the adaptive cross approximation (ACA) method in conjunction with an iterative matrix solver to reduce the memory and complexity requirements, allowing this tool to be applied to very large problems.

Simulation results for a 900 MHz blade antenna on an unmanned drone
You can link HFSS designs as sources in an HFSS-IE design using a data link. The feed can be created in HFSS and then fields from that simulation can be linked into the target HFSS-IE design with a few mouse clicks. In addition, you can also include the feed structure of the source simulation in the target HFSS-IE design and include its scattering in the final result.
In combination with HFSS the design engineer can now pick the best solver to use for a given situation and in many cases can take advantage of both solvers in a linked project.

Currents on a reflector antenna modeled in HFSS-IE with the source simulated in a separate HFSS design. The simulation accurately predicts the induced currents on the feed structure.