Because 3D FEM calculations are usually much slower than mode matching calculations, Mician has introduced an advanced mode matching technique which is capable of considering losses by approximation. This feature has been implemented to nearly all elements of the iris and steps library.  
Example for computation on a waffle iron filter: total computation time for 500 frequency steps: 16 seconds for “ideal” and 17 seconds for “invar” by normal sweep and 7 seconds for “ideal” and 8 seconds for “invar” by adaptive sweep. (Pentium IV, 2.8GHZ).

Additionally it is possible to use 3D FEM on circuit level for a rigorous inclusion of losses - either dielectric or arising from finite wall conductivity -into the calculation.