[Python] Scipy OptimizeWarning: Covariance of the parameters could not be estimated when...

I'm trying to plot some data with a non-linear fit using the function:

kB and Bv being a constant while J'' is the independent variable and T is a parameter.

I tried to do this in Python:

#Constants
kb = 1.380649e-23
B = 0.3808

#Fit function
def func(J, T):
return (B*(2*J+1) / kb*T * np.exp(-B*J*(J+1)/kb*T))
popt, pcov = optimize.curve_fit(func, x, y)
print(popt)

plt.plot(x, y, "o")
plt.plot(x, func(j, popt[0]))


But this results in the warning "OptimizeWarning: Covariance of the parameters could not be estimated warnings.warn('Covariance of the parameters could not be estimated'" and the parameter turns out to be 1.

This is the data:

x = [2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48]
y = [0.185,0.375,0.548,0.695,0.849,0.931,0.996,0.992,1.0,0.977,0.927,0.834,0.691,0.68,0.575,0.479,0.421,0.351,0.259,0.208,0.162,0.135,0.093,0.066]




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