Is it possible to derive new output from other two outputs in Field output. I found it is impossible from the error message attached. 

I wanted to see contour of the product of Energy and thickness of shell. I think it is very simple operation but why not possible ?

I could create  output     sqrt(E)     E:energy 

but not  sqrt(E) * thickness  

It looks simple operation but why not possible ? Is there any way to create my own output like this ?