hfss material

I want to assign some material for Ground or Air.
But I am always confused by material perpeties.
such as
material of "air" & "vaccum" , which is suitable for air box ?
PEC material or Perfect E boundaries, which can used in Ground plane?

thanks

I use air instead of vaccum, copper instead of PEC. But maybe the difference is very small.

@filtronetics: The difference is more significant as the increasement of desired frequency.
@myem: Using 'air' or 'vacum' for airbox in HFSS doesn't matter.
PEC material is used for 3D objects, Perfect E boundaries is prefered to use for 2D objects.
It's the best way to simulate your ground structure with 3D object and assign its material as Copper or some others that are used in fact. (even PEC material if you want)
In the case of simulating your 2D ground structure, ofcourse, assign it as Perfect E boundaries.
Cheers!