Converting Matlab to C

So I have solved a problem in matlab and it works brilliantly. However my boss requires the method in C. I am doing a least squares minimization using fminsearch. I am trying to replicate the function being solved for in C (I already have the neldermead function in C) My matlab function to be solved is:

function sse=myfit(params,Input,Actual_Output)
global c
a=params(1);
b=params(2);

x = (exp(a*2*pi)-cosh(a*b*Actual_Output))/sinh(a*b*Actual_Output);
c = -1/atanh(x);
Fitted_Curve = (1/(a*b))*(c-asinh(sinh(c)*exp(a*Input*2*pi)));
Error_Vector=Actual_Output-Fitted_Curve ;
sse=sum(Error_Vector.^2);

And this is what I am trying to replicate in C:

static double function(int n, double x[])
{
  double c, x_coeff;
    double Fitted_Curve[5];
    double Error_Vector[5];
    int i;
    double Actual_Output[5]={1.2, 2.693, 4.325, 6.131, 8.125};
  double Input[5]={1, 2, 3, 4, 5};
  double sum = 0;

x_coeff = (exp(x[0]*2*pi)-cosh(x[0]*x[1]*Actual_Output[0]))/(sinh(x[0]*x[1]*Actual_Output[0]));
c = (-1)/(atanh(x_coeff));
  for (i = 0; i <= 4; i++)
  {

      x_coeff[i] = (exp(x[0]*2*pi)-cosh(x[0]*x[1]*Actual_Output[0]))/(sinh(x[0]*x[1]*Actual_Output[0]));

      c[i] = (-1)/(atanh(x_coeff[i]));

      Fitted_Curve[i] = (1/(x[0]*x[1]))*(c[i]-asinh(sinh(c[i])*exp(x[0]*Input[i]*2*pi)));

      Error_Vector[i] = Actual_Output[i]-Fitted_Curve[i];

   printf(" x_coeff(%d) =   %f  %f\n", i, x_coeff[i], c[i]);
      }

      for (i = 0; i <= 4; i++)
      {
          sum = sum + Error_Vector[i]*Error_Vector[i];
      }

      return sum;

}

It doesnt give the same answer (atanh(x>1) = inf) which obviously suggests they are not doing the same thing. Can anyone see where I have gone wrong?

Thanks