Which EM tool is most memory effecient

ANSOFT Designer(Ensemble):
Ansoft designer SVD(singular value decomposition) fast solve technology claims to be able to simulate 30,000 unknowns with 2 GB Ram.
IE3D 11.06:
Using GEMS(generally effeceint matrix solver)claims to solve 63,597 unknowns with 548 MB RAM.(IEEE Ant. & Prop. magzine june 2005)
What is the formula for relating "unknown and memory" for both ansoft designer planer EM solver and IE3D11.06?????????????.The conventinal MOM solvers like momentum can solve much less unknowns with 2GBs RAM limit (for single application) on 32 OS.I am currently using momentum for which unknowns and memory is related as follow:
ADS Momentum:
When "n" is the number of unknowns, then Momentum needs to allocate approximately memory= 16 bytes * sqr(n).
For 2 GB this means 11585 unknowns.

FMM is used to modify the 3-D scatter, the unknowns can reach 8,000 ,000

zubair: could you please tell us the title of the paper from which the above IE3D data is quoted?

Feko has the (ML)FMM implemented. so maybe Feko is the one--according to your standard--for the restricted types of problems the algorithm can handle.

It is an advertizement from Zeland located on the backside of front cover in:

"IEEE Antenna and propagation magazine"Vol. 47,No.3,June 2005

and regarding Ansoft designer/ensemble the paper is

"New planar field solver for very large and very small structures",By Lawrence Williams, Ph.D.,Brian Gray, Tony Donisi, and Richard Hall, Ph.D

located at www.rfdesign.com