As it stands, the starting parameters are bad and the model has no variance over the range, try
Plot[model[x, 37., 1.68, 50., 0.46, 0.73], {x, 1.63, 1.71},
Epilog -> {PointSize[Small], Point[Data4kI3]}, PlotRange -> Automatic]
and see that the model grows seemingly linear between
In[60]:= model[#, 37., 1.68, 50., 0.46, 0.73] & /@ {1.63, 1.71}
Out[60]= {3.21643, 3.21647}
3.21643 and 3.21647, but the data vary in y from 0.56 to 0.63 over the range and have a strong peak around x = 0.65.
Look at this
Plot[model[x, 37., 1.68, 50., 0.01, 0.55], {x, 1.63, 1.71},
Epilog -> {PointSize[Small], Point[Data4kI3]}, PlotRange -> {{1.63, 1.71}, {0.56, 0.64}}]
to estimate how bad (without variability) the model is over the x-range the data have. Therefore NonlinearModelFit[] gives nothing but a horizontal line at about y=0.601 back.
What to do next:
- Check the model (typos, missing expressions, ...)
- find a parameter area where it has the variability which is necessary to fit the data
- if such an area of parameters is not available: change the model
- return to NonlinearModelFit[] and get the job done
