What Keymaster means is that what's in CoronaMTL is identical to what's in VolumeMTL.
If you want to get identical results to VolumeMTL here are two ways:
1, Set opacity level to 0.0 - this will make sure CoronaMTL will behave and perform same as VolumeMTL
2, Instead of 0 opacity, set refraction level to 1.0, then set IOR to 1.0, and set diffuse level to 0.0. This will perform slightly slower than 0 opacity setup, but parameters like reflection or refraction glossiness will have an effect. Zero opacity will completely ignore diffuse, reflection and refraction.