Curve Fitting for Temperature and specific heat values using Python


The key objective of writing this program is to perform curve fitting with the help of a linear and cubic polynomial for the Temp. & Cp data.

#1. What does popt and pcov mean?

#2. What does np.array(temperature) do?

#3. What does the * in *popt mean?

#4. Write code to fit a linear and cubic polynomial for the Cp data?

#5. What needs to be done in order to make the curve fit perfect?


Curve fitting used to find the "best fit" line or curve for a series of data points.
Process of constructing a curve or mathematical functions that gives the closest proximity to the real series of data. It is highly effective in mathematical modeling for some processes.

#1. popt An array of optimal values for the parameters which minimize the sum of squares of residuals pcov-2d array which contains the estimated covariance of popt. The diagonals provide the variance of the parameter estimate. And, the curve fit function returns two values, popt and pcov popt returns the value if co-efficient of the polynomial used and pcos refers to the co-variance of these coefficients.

#2. np.array(temperature) it is an array to store values of different temperature. Basically, it saves all the values of temperature in an array from the sequence, so all the values can be called for each value of cp at every temperature.

#3. *popt is used to refer to the coefficient values from the array. The * is used to call the arguments. Instead of calling each coefficient separately, *popt can be used to call all the coefficient in one instance.


1. We have cp vs temp. data file.

2. Open a sublime file save it with a proper name that having .py extension.

3. Import all the necessary modules;

  • import matplotlib.pyplot as plt
    This module helps us to plot the graph of the given result.
  • import math
    This module helps us to use mathematical functions like pie, sine, cos, etc.
  • import numpy
    This module helps us to do mathematical operations on an element of an array.
    Whenever we want to define more than one value in a single variable we have to use an array and numpy.linspace is also the same thing it stores value in a range like this; numpy.linspace (lower limit, upper limit, no. of division)
    This works acc. to a formula:
    Size of division = (upper limit-lower limit)/(no. of division-1)
  • From scipy.integrate import odeint
    Odeint function which integrates the system of ODE.

3. Use an empty array for storing a value that we get from FOR loop.

4. Append is a command that will continuously append the value of a variable. It keeps on adding the values into an existing array.

5. Define the function: We can call the function whenever and wherever we required.

def func(t,a,b):  here t is a variable and a, b is constant in linear equation.

Define another function read_file():
Create the FOR loop for reading the file which we have in our database by the name of 'Data' which contains temperature and specific heat values. we append those all values into an empty array of name temperature and cp.

6. The main program for the linear function

we call the function  read_file() and here we use popt (function gives the value) & pcov (function gives the square matrix).

fit_cp is a new array to stores the value of new specific heat.
Here *popt helps to make arguments from an array.

Now plot the curve for the linear equation by using plt.plot command and mark labels also for that curve. We can change the color and linewidth of our curve also.

7. Done it the same thing for the cubic equation: define a new function and name them accordingly and make for loop and call that function for work. Plot the curve for the cubic equation and see the difference.

8. Now we find the error in both linear and cubic separately. For that, we take some formulas for our given data. For choosing the best fit.

SSE = the sum of square due to errors

`R^2`= SSR/SST
R Square measures how successfully the fit is explaining the variation of data. it ranges from 0 to 1.
0 is the worst possible fit for the data set
1 is the best possible fit for the data set

As a result, we give only `R^2` & RMSE values by which we can define which curve is best suited for us.

Now plot the graph for the cubic equation and observe what results we get.

import matplotlib.pyplot as plt 
import math
from scipy.optimize import curve_fit
import numpy as np

#curve fit linear function
def func(t,a,b):
	return a*t + b

#reading thermodynamic data file
def read_file():
	temperature = []
	cp = []
	for line in open('data' , 'r'):
		values = line.split(',')
	return [temperature, cp]

# Main program for linear function
temperature, cp = read_file()
popt, pcov = curve_fit(func, temperature, cp)
fit_cp = func(np.array(temperature), *popt)  			# here * helps to make arguments from array

plt.plot(temperature,cp, color='blue' ,linewidth = '3')   		 #ideal curve we get 
plt.plot(temperature,fit_cp, color = 'red' , linewidth= '3')	#actual curve we get
plt.legend(['actual data' , 'curve fit'])
plt.xlabel('temperature [k]')

#curve fit cubic funtion 

def func3(t,a,b,c,d):			# 3 denotes cubic
	return a*pow(t,3)+b*pow(t,2)+c*t+d

#Main program for cubic function
temperature, cp = read_file()
popt3, pcov3 = curve_fit(func3,temperature,cp)
fit_cp3 = func3(np.array(temperature), *popt3)

plt.plot(temperature,cp, color='blue' ,linewidth = '3')
plt.plot(temperature,fit_cp3, color='red' ,linewidth = '3')
plt.legend(['actual data' , 'curve fit'])
plt.xlabel('temperature [k]')


#----------ERROR IN LINEAR

##for SSR
j= np.mean(cp)

jarray=[j] * len(fit_cp)
sub = (fit_cp - j)
subsq= np.square(sub)
SSR= sum(subsq)

##for SSE
sub1= (cp - fit_cp)
sqsub1= np.square(sub1)
SSE= sum(sqsub1)


Rsq = SSR /SST

RMSE = math.sqrt(SSE/len(fit_cp))

#----------ERROR IN Cubic

sub3 = (fit_cp3 - j)
subsq3= np.square(sub3)
SSR3= sum(subsq3)

##for SSE
sub1_3= (cp - fit_cp3)
sqsub1_3= np.square(sub1_3)
SSE3= sum(sqsub1_3)


Rsq3 = SSR3 /SST3

RMSE3 = math.sqrt(SSE3/len(fit_cp3))


Curve fit for the Linear polynomial and observe what results in we get.

Curve fit for the cubic polynomial and observe what results in we get.


1. As the seen plot of the linear and cubic polynomial, the cubic polynomial is closest to the function curve.

2. #5. In order to make or have a better curve fit we have to use higher-order polynomial which is used to fit closest to the function curve (reduces the deviation from data). Therefore we can say that polynomial of higher degree provides a better curve fit.

3. Here we learn how to plot a graph from the given data file which is in our database.

Google Drive link for 'data' file:


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