On page 137 of the 2026/R1 PDF manual, Zemax describes this situation for determining the semidiameter of a mount in a thermal model: 

OpticStudio handles this situation in one of two ways. If, due to thermal expansion, the radial position of the contact point were to become smaller than the lens mechanical semi-diameter, then the thermally-adjusted radial size of the mount is used in calculating the "edge thickness". However, if the reverse is true and the mount radial contact points were to thermally expand to fall outside the lens mechanical semi-diameter, then OpticStudio instead uses the lens mechanical semi-diameter in calculating the "edge thickness"This behavior highlighted below means the changes for + or – temperature changes are not symmetrical, if you have a lens where the distance after the 2nd vertex of the lens defines the spacer.

What this means is that Zemax adjusts the size of the mount in a way so the behavior is not symmetrical for small ± changes in temperature around the nominal operating point. This is non-physical, but they do it to avoid the case of the lens “falling into” the spacer. Ok, I get why they did that.  But it does not model reality.  

I do the work around of this, let’s take a simple example:

A lens is followed by a spacer.  The V2 radius is 100 mm.  I put a dummy surface that is 0.00 mm after V2, with a radius of 100 mm. I then make the aperture of that dummy surface to be (say) 0.5 mm less than the lens diameter. This simulates the contact point of the spacer following V2. I set the TCE of that following surface to the spacer material (say aluminum). 

Is there any way around the labor of inserting these dummy surfaces?  Am I missing something?  (You also have to do it on the front of lenses, where a spacer precedes a lens).  It would be great is there was someway in Zemax to track these mechanical diameters. 

Thanks for your time.