Hi Thanks a lot Scott , I got that how to change ss for difeerent path by using the function debye(temp, thetad).But I am not getting the same result in case of delr . I took the different parameter as follow : guess amp 1 def delr alpha*reff guess enot 0 def ss debye(temp,thetad) set alpha .0002 set thetad 343 set temp 298 But I am not getting a very good fit. The value of debye temperature is 343 for Cu( Kittel book) but I got to know that some persons even guessed the value of thetad as 285+-16 . I am not getting that how we can guess the value of alpha and thetad . Also I am not getting the value of Ei, 3rd and 4th cumulant in the fit results . So How can I get the value of these parameters in the fit results . With thanks Abhijeet Gaur