function diff = fit_simp_alkili_practice(x,X,Y)
% This function is called by LSQNONLIN.
% x is a vector which contains the coefficients of the
% equation. X and Y are the option data sets that were
% passed to lsqnonlin.
 
A=x(1);
B=x(2);
C=x(3);
Z=C./X;
for k=1:5
diff = diff + A*B/(C^6)*X.^5.*(120*zeta(5)-exp(-1.0*k.*Z).*(Z.^5+(5/k)*Z.^4+(20/k^2).*Z.^2+(120/k^4).*Z+120/k^5));
end
diff = diff - Y;
end
