The problem I see is that these are not optical material properties, per se. However, we have no other places where we define material properties. If these are the only non-optical material properties throughout nexus, maybe we can leave them where they are now.

Actually, the name OpticalMaterialProperties is misleading. In G4 these are just generic material properties tables. It just happens that most of them are optical stuff. For example, the scintillation parameters are also stored in these tables, which one may say has to do with optical properties, but not really (it doesn't say anything about how the photons interact with the material). I believe the material properties tables are just a very generic way for the user to hold any data related to the material and use it elsewhere in the physics code.

Ok, so are you proposing to change the name to MaterialProperties, generically? I think it makes sense.

It would be technically more correct and probably good enough to not confuse users.