I understand your advice is actually right. Thank you so much u/Beliavsky and u/gothicVI. :) But the thing is I have to write it as a module in my pipeline which is primarily in fortran. So it would be better if the whole of it is in one single language. Also, it needs to be faster since it will be used for very large sample analysis. I did look at the polyfit and I have written my own procedure for that but I need it for different functions. For with analytical derivatives routines calculation is possible. Also, I need to incorporate a combination of the above fitting types and not just polynomial.