Can't fit data to given function with scipy on python (Jupyterlab)

The problem is that I need to fit some data into a function (which was given by my professor, so basically I can't change it) but the fit goes poorly and I can't understand why. I also tried with lmfit method but that didn't work too. Here is the code I used and the image of the fit I get with residuals:Fit

x1 = np.array([200.  , 220.  , 240.  , 260.  , 280.  , 300.  , 320.  , 340.  ,
360.  , 380.  , 390.  , 392.5 , 395.  , 397.5 , 400.  , 402.5 ,
405.  , 407.5 , 410.  , 411.25, 412.5 , 413.75, 414.  , 415.25,
416.5 , 417.75, 418.5 , 420.  , 422.5 , 425.  , 427.5 , 430.  ,
432.5 , 435.  , 437.5 , 440.  , 450.  , 460.  , 480.  , 500.  ,
510.  , 520.  , 540.  , 560.  , 570.  , 580.  , 600.  ])
y1 = np.array([0.00787402, 0.00904762, 0.01079365, 0.01269841, 0.01539683,
0.01857143, 0.02321429, 0.0302    , 0.04303279, 0.06767896,
0.09456265, 0.10338983, 0.11410579, 0.12545932, 0.14700855,
0.16944444, 0.19692833, 0.22965779, 0.27434783, 0.28681818,
0.32079208, 0.32562814, 0.32893401, 0.33061224, 0.32727273,
0.31268293, 0.30236967, 0.28940092, 0.24274194, 0.20989011,
0.18052805, 0.15531915, 0.13259053, 0.11627907, 0.10341463,
0.07345133, 0.07455357, 0.06403509, 0.04163265, 0.03811475,
0.02948207, 0.0272    , 0.02301587, 0.02027559, 0.01968254,
0.01795276, 0.01672584])
σ_y1 = np.array([0.00021564, 0.00023222, 0.00025707, 0.00028599, 0.00032924,
0.00038243, 0.00046297, 0.00058876, 0.0008021 , 0.00127062,
0.00180749, 0.00196919, 0.00217276, 0.00239208, 0.00284333,
0.00317296, 0.00368032, 0.00432658, 0.00507593, 0.00532676,
0.0060111 , 0.00611128, 0.00618   , 0.00621495, 0.00607953,
0.0057964 , 0.0056074 , 0.00536185, 0.00463023, 0.00401191,
0.00347255, 0.00303842, 0.00250275, 0.00219007, 0.00196289,
0.0014738 , 0.00139797, 0.00129955, 0.00077378, 0.00082702,
0.00054713, 0.00053481, 0.00043752, 0.00041071, 0.0003831 ,
0.00037095, 0.00033556])
# Primo test di fit con 4 parametri
def fitfunc1(x, A, R, L, C, Off): # le frequenze 'f' sono in kHz
ω = 2*np.pi*x 
δ = R/(2*L)
Ω = 1/sqrt(L*C)
return A*ω/sqrt(ω**4-2*ω**2*(Ω**2-2*δ**2)+Ω**4)
R_tot = 77.54
R_G = 48
L = 308*1E-6
C = 508*1E-9
p_init1 = [31454, R_tot-R_G, L, C] # valori iniziali di (A, R, L, C)
p_best1, cov1 = curve_fit(
fitfunc1, x1, y1,          # assegno funzione di fit, ascisse e ordinate
sigma=σ_y1,                     # assegno gli errori sulle ordinate
p0=p_init1, bounds=(0, +np.inf)  # imposto i valori iniziali dei parametri e intervalli ammessi [0, +∞) 
)                                 
#Somma dei residui e deviazione standard a posteriori
res1 = (y1-fitfunc1(x1, *p_best1))
sum_res1 = sum(res1)
σ_post1 = ma.sqrt(sum_res1/(y1.size))
print("---------------------------")
print("Best fit values:")
print("---------------------------")
print("A = (%.5f +/- %.5f)   err_percentuale = %.5f" % (p_best1[0],sqrt(cov1[0,0]),sqrt(cov1[0,0])*100/p_best1[0]))
print("R = (%.5f +/- %.5f) Ω err_percentuale = %.5f" % (p_best1[1],sqrt(cov1[1,1]),sqrt(cov1[1,1])*100/p_best1[1]))
print("L = (%.5f +/- %.5f) μH err_percentuale = %.5f" % (p_best1[2]*1E+6,sqrt(cov1[2,2])*1E+6,sqrt(cov1[2,2])*100/p_best1[2]))
print("C = (%.5f +/- %.5f) F err_percentuale = %.5f" % (p_best1[3]*1E+12,sqrt(cov1[3,3])*1E+12,sqrt(cov1[3,3])*100/p_best1[3]))
print("---------------------------")
fig1 = plt.figure(1, figsize=(9,7))
frame1a=fig1.add_axes((.1,.3,.7,.5))
# Grafico del fit
plt.errorbar(x1, y1, yerr=σ_y1, fmt='.', label='dati', capsize = 4)
_pts = np.linspace(x1[0], x1[-1], 1000)
plt.plot(_pts, fitfunc1(_pts, *p_best1), label='fit')
plt.title("Amplificazione ai capi di R")
plt.xlabel("frequenza [kHz]")
plt.ylabel(r"A = $V_{out}/V_{in}$")
plt.legend()
plt.tick_params(bottom = False)
plt.grid()
frame1a.set_xticklabels([])
# Grafico dei residui
frame1b=fig1.add_axes((.1,.1,.7,.19))
plt.errorbar(x1, res1, yerr = σ_y1, fmt ='.', capsize = 4)
plt.xlabel("frequenza [kHz]")
plt.ylabel(r"$A_{mis}-A_{fit}$")
plt.grid()
#salvataggio della file nella cartella, dpi = risoluzione immagine, bbox_inches = risolve problema di dimensione
#plt.savefig("name_of_file", dpi = 200, bbox_inches = "tight")

I tried several things like removing the bounds for the parameters but that gives me enormous errors, renaming the parameters (in case they conflicted with the name of the initial value) and varying the initial values to let the fit be more precise (however initial parameters should be fixed as I wrote them because those are values that were measured with particular devices). (R_tot-R_G) represents the resistance of our RLC circuit, L is the inductance, and C is the capacity. The only unknown value is the amplitude A of the curve. Thank you in advance!

A simple method to get very good approximates of the parameters :

Now if you want more accurate results according to some criteria of fitting (LMSE, LMSRE, LMAE or other) you can use the above approximates values of parameters as initial values to start an iterative method of non-linear regression (with python or any other convenient software).

I think that your script is a very good start, but there are two separate issues with your fit. First, I think that you will find that for a single oscillator (and, just your mathematical function) that R, L, and C will not be able to be all determined independently -- they are almost completely correlated. Fixing one of these values will allow the other two to be determined well.

Second, the initial values you used for R, L, and C are so far from optimal that the fit simply cannot find a better solution. I think the simplest way to view that is that your initial value of L is too low by a factor of 1000.

Initial values always matter for a fit. A related topic (or perhaps a third topic) is that you should put lower bounds of 0 on all of R, L, and C -- that's physically justifiable but also avoids problems with taking the square root of a negative number.

Using lmfit (as I am a lead author -- I'm sure this could be done with curve_fit and give nearly identical results), a fit allowing all of R, L, and C, as well as A and Off (added to your function) would look like:

import numpy as np
import matplotlib.pyplot as plt
import lmfit
def fitfunc1(x, A, R, L, C, Off): # le frequenze 'f' sono in kHz
ω = 2*np.pi*x
δ = R/(2*L)
Ω = 1/np.sqrt(L*C)
return Off + A*ω/np.sqrt(ω**4-2*ω**2*(Ω**2 - 2*δ**2)+Ω**4)
model = lmfit.Model(fitfunc1)
params = model.make_params(A=40, R=30, L=0.30, C=0.5e-6, Off=0)
params['A'].min = 0
params['R'].min = 0
params['L'].min = 0
params['C'].min = 0
# params['L'].vary = False
init = model.eval(params, x=x1)
result = model.fit(y1, params, x=x1, weights=1/σ_y1)
print(result.fit_report())
# #Somma dei residui e deviazione standard a posteriori
res1 = y1 - result.best_fit
fig1 = plt.figure(1, figsize=(9,7))
frame1a=fig1.add_axes((.1,.3,.7,.5))
#
# Grafico del fit
plt.errorbar(x1, y1, yerr=σ_y1, fmt='.', label='dati', capsize = 4)
plt.plot(x1, result.best_fit, 'o', label='best fit')
_pts = np.linspace(x1[0], x1[-1], 1000)
plt.plot(_pts, result.eval(x=_pts), label='interpolated fit')
plt.title("Amplificazione ai capi di R")
plt.xlabel("frequenza [kHz]")
plt.ylabel(r"A = $V_{out}/V_{in}$")
plt.legend()
plt.tick_params(bottom = False)
plt.grid(color='#EEE')
frame1a.set_xticklabels([])
#
# ....

would give a fit report of

[[Model]]
Model(fitfunc1)
[[Fit Statistics]]
# fitting method   = leastsq
# function evals   = 65
# data points      = 47
# variables        = 5
chi-square         = 400.284733
reduced chi-square = 9.53058888
Akaike info crit   = 110.675341
Bayesian info crit = 119.926079
R-squared          = -673.477539
[[Variables]]
A:    30.9354753 +/- 0.67812562 (2.19%) (init = 40)
R:    23.3162313 +/- 1003.45159 (4303.66%) (init = 30)
L:    0.25082189 +/- 10.7859228 (4300.23%) (init = 0.3)
C:    5.8633e-07 +/- 2.5214e-05 (4300.36%) (init = 5e-07)
Off:  6.8886e-04 +/- 4.3907e-04 (63.74%) (init = 0)
[[Correlations]] (unreported correlations are < 0.100)
C(L, C)   = -1.0000
C(R, L)   = +1.0000
C(R, C)   = -1.0000
C(A, Off) = -0.8021
C(A, R)   = +0.5709

showing huge uncertainties for R, L, and C because their correlations all exceed 0.999.

Simply fixing L (by uncommenting # params['L'].vary = False) would give a fit with the same value for chi-square and a report of

[[Model]]
Model(fitfunc1)
[[Fit Statistics]]
# fitting method   = leastsq
# function evals   = 26
# data points      = 47
# variables        = 4
chi-square         = 400.284733
reduced chi-square = 9.30894728
Akaike info crit   = 108.675341
Bayesian info crit = 116.075931
R-squared          = -673.477539
[[Variables]]
A:    30.9354748 +/- 0.55046897 (1.78%) (init = 40)
R:    27.8877948 +/- 0.84442502 (3.03%) (init = 30)
L:    0.3 (fixed)
C:    4.9022e-07 +/- 5.3035e-10 (0.11%) (init = 5e-07)
Off:  6.8887e-04 +/- 3.9717e-04 (57.66%) (init = 0)
[[Correlations]] (unreported correlations are < 0.100)
C(A, R)   = +0.7666
C(A, Off) = -0.7613
C(R, Off) = -0.5502

FWIW, the plot using the above modifications of your code would look like