Using dependent variable to calculate a material property

Hi,

I am rookie when it comes to comsol but I have simple question. I would like to use a dependent variable (potential) to calculate a material property ( such as density of states). So for each iteration performed by comsol a value for the potential is calculated. I want to use this value to calculate a material property. I have been trying to use the analytic function to make an attempt to do this but I have had no success. I appreciate any help I can get.