Complementary error function curve fitting

Hi,

I have an experimental dataset which I know would be fitted by erfc(x)

I tried using the custom equation option in the curve fitting toolbox, but it does not work.

The generated code looks like this

ft=fittype('A*erfc(2e5*sqrt(a*x))+c', 'independent', 'x', 'dependent', 'y' );

fitresult=fit(x,y,ft);

plot(fitresult,x,y)

and the figure after fitting

I do not know where I am going wrong.

Answer 1

[Star Strider]

You likely need to allow the function to find its own centre:

ft=fittype('A*erfc(2e5*sqrt(a*(x-b)))+c', 'independent', 'x', 'dependent', 'y' );

Your equation assumes your data are centred about x=0. Changing the expression from ‘a*x’ to ‘a*(x-b)’ allows the function to centre around b. I can’t test your code, but that should work.

[Nachiketa]

Thanks for the reply!

x-b does not work (since its not real)

I tried x+b... but I still get the above figure without any fitting.

[Star Strider]

My pleasure!

Actually this is what I intended:

ft=fittype('A*erfc(2e5*sqrt(a*x)-b)+c', 'independent', 'x', 'dependent', 'y' );

I initially put b in the wrong place in the erfc argument. My apologies. (I was up late last night fighting a spambot here on MATLAB Answers.)