From cb095f3fa9b2ec941bcd9447b84da10c77788a97 Mon Sep 17 00:00:00 2001
From: Pim Nelissen <pim.nelissen@protonmail.com>
Date: Mon, 27 Oct 2025 15:40:55 +0100
Subject: [PATCH] task 3 solution

---
 III.py | 121 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)
 create mode 100644 III.py

diff --git a/III.py b/III.py
new file mode 100644
index 0000000..4011a12
--- /dev/null
+++ b/III.py
@@ -0,0 +1,121 @@
+import numpy as np
+from matplotlib import pyplot as plt
+
+from RubberBand import RubberBand
+
+def generate_ensemble(f):
+    """
+    generate an ensemble of RubberBands with a force f applied
+    """
+    
+    lengths = np.zeros((NUM_BANDS,))
+    
+    for i in range(NUM_BANDS):
+        band = RubberBand(N, a=a, kBT=kBT, force=f)
+        lengths[i] = band.length/band.a
+    
+    return lengths
+
+def calculate_k_eff_theory(N, a, kBT):
+    return N*a**2/kBT
+    
+def true_expectation_values(f, N, a, kBT):
+    """
+    compute 'true' expectation value <L> for both full analytical one and
+    using the small force limit for the linear region
+    """
+    L_analytical = N*a*np.tanh(a*f/kBT)
+    L_analytical_linear = f*calculate_k_eff_theory(N, a, kBT)
+    
+    return L_analytical, L_analytical_linear
+
+def plot_true_L_vs_sim_L(forces, means, stdevs, params):
+    L_true, L_true_linear = true_expectation_values(forces, *params)
+
+    fig, (ax1, ax2) = plt.subplots(2, 1, figsize=(5, 9))
+    
+    # Plot 1: true L vs approx L in entire force range
+    ax1.plot(forces, L_true, color='red',
+             label="$\langle L \\rangle$ (analytical)")
+    # ax1.plot(forces, L_true_linear, color='red', alpha=1,
+             # label="$\langle L \\rangle$ (analytical, small force approximation)")
+    ax1.plot(forces, means-stdevs, color='blue', alpha=.5)
+    ax1.plot(forces, means+stdevs, color='blue', alpha=.5)
+    ax1.fill_between(forces, means-stdevs, means+stdevs, color='blue', alpha=0.1)
+    ax1.plot(forces, means, color='blue', linestyle=':', linewidth=2,
+             label=f"$\langle \hat{{L}} \\rangle$ (M={NUM_BANDS:.0E})")
+    ax1.set_xlabel("$f$ [arb.]")
+    ax1.set_ylabel("$\langle L \\rangle (f)$")
+    ax1.legend()
+    
+    # Plot 2: true L vs approx L in small force range
+    ax2.plot(forces, L_true_linear, color='red',
+             label="$\langle L \\rangle$ (small force approximation)")
+    ax2.plot(forces, means-stdevs, color='blue', alpha=.5)
+    ax2.plot(forces, means+stdevs, color='blue', alpha=.5)
+    ax2.fill_between(forces, means-stdevs, means+stdevs, color='blue', alpha=0.1)
+    ax2.plot(forces, means, color='blue', linestyle=':', linewidth=2,
+             label=f"$\langle \hat{{L}} \\rangle$ (M={NUM_BANDS:.0E})")
+    ax2.set_xlim(0, 0.5)
+    ax2.set_ylim(0, 75)
+    ax2.set_xlabel("$f$ [arb.]")
+    ax2.set_ylabel("$\langle L \\rangle (f)$")
+    ax2.legend()
+    plt.tight_layout()
+    
+    # plot 3: abs difference between true and approx L
+    plt.figure()
+    plt.plot(forces, np.abs(L_true-means), 
+             label="$\langle L \\rangle$ (analytical)")
+    plt.plot(forces, np.abs(L_true_linear-means),
+             label="$\langle L \\rangle$ (small force approximation)")
+    plt.yscale('log')
+    plt.xlabel("$f$ [arb.]")
+    plt.ylabel("$| \langle L \\rangle - \langle \hat{L} \\rangle |$")
+    plt.legend()
+    plt.tight_layout()
+    
+    plt.show()
+    
+def estimate_k_eff(forces, means, upper_lim=0.5):
+    x = forces[forces < upper_lim]
+    y = means[forces < upper_lim]
+    m, b = np.polyfit(x, y, 1)
+    return m
+
+if __name__ == "__main__":
+    # simulation parameters
+    USE_SAVED = True
+    NUM_BANDS = int(1E3)
+    N = 100
+    a = 1.
+    kBT = 1.
+    
+    params = (N, a, kBT)
+    
+    forces = np.arange(0., 5, 0.01)
+    
+    if USE_SAVED:
+        means = np.load("data/task3_means.npy")
+        stdevs = np.load("data/task3_stdevs.npy")
+    else:
+        means = np.zeros(len(forces))
+        stdevs = np.zeros(len(forces))
+        
+        for i, f in enumerate(forces):
+            L_i = generate_ensemble(f)
+            means[i] = np.mean(L_i)
+            stdevs[i] = np.std(L_i)
+            
+        np.save("data/task3_means.npy", means)
+        np.save("data/task3_stdevs.npy", stdevs)
+    
+    # generate the ensembles and compute values
+    plot_true_L_vs_sim_L(forces, means, stdevs, params)
+    
+    lim = 0.3
+    k_eff_est = estimate_k_eff(forces, means, upper_lim=lim)
+    print(f"""Analytical k_eff v.s. estimate from fit:
+          analytical (N * a^2/kBT):     k_eff = {calculate_k_eff_theory(*params)}
+          estimate (f < {lim}):         k_eff = {round(k_eff_est, 2)}
+          """)
\ No newline at end of file