Van der Waals equation - Newton's method

Hi

I'm stuck and I keep getting this error

Not enough input arguments.

Error in fun (line 4)
F(1) = log((3*x - 1)/(3*y - 1)) + 9/(4*T) * (1/x - 1/y) - 1/(3*x - 1) + 1/(3*y - 1);

Error in newton_ndim (line 6)
resd = [norm(feval(F,xk))];

Error in practica5 (line 16)
    [XK,resd,n]=newton_ndim(@fun,X,tol,itmax);

Here is my main script

X = [1.2 ; 0.8];
Vk = [];


tol = 1e-16;
itmax = 100;


for T = 0.99:-0.01:0.85
    
    [XK,resd,n]=newton_ndim(@fun,X,tol,itmax);
    
    Vk=[Vk XK(:,end)];
    
    X = XK(:,end);

end

The function I'm trying to solve is this one, obtained from Van der Waals equation

function F = fun (X,T)

x = X(1); y = X(2);
F(1) = log((3*x - 1)/(3*y - 1)) + 9/(4*T) * (1/x - 1/y) - 1/(3*x - 1) + 1/(3*y - 1);
F(2) = (8*T)/3 * (1/(3*x-1) - 1/(3*y-1)) - 1/x^2 + 1/y^2;
end

Do you know how to solve this?

Answer 1

[Are Mjaavatten]

In your code, newton_ndim does not know the value of T and so cannot call fun(X,T). You can probably fix this by defining an inline function (here called ffun) inside your loop:

for T = 0.99:-0.01:0.85
    ffun = @(X) fun(X,T);
    [XK,resd,n]=newton_ndim(ffun,X,tol,itmax);
    Vk=[Vk XK(:,end)];
    X = XK(:,end);
end

[Blanca Mir Pou]

Thank you, it works