“If I’ll not use Method->”EquationSimplification”-> “Residual”, will have some addtional error as well.” Needs[“VectorAnalysis`”] f = 1/2 K1 Dot[s, s] + 1/2 K2 (t)^2 + 1/2 K3 Dot[b, b] – (ξo ξv (E1)^2 )/ 2 – (ξo ξa (E1)^2 (n)^2)/2 n = {Cos[θ[z]], 0, Sin[θ[z]]}; s = n Div[n, Cartesian[x, y, z]]; t = Dot[n,Read more