update link and rubberband structures

Link.py

@@ -8,4 +8,4 @@ class Link:
             direction (int) The direction. Must be 1 or -1.
         """
 
-        self.direction = direction
+        self.direction = direction

RubberBand.py

@@ -3,7 +3,10 @@ import numpy as np
 from Link import Link
 
 class RubberBand:
-    def __init__(self, N, a=1.):
+    # class attributes
+    k_B = 1
+    
+    def __init__(self, N, a=1., T=1.):
         """
         RubberBand is a class that can simulate a rubber band with N
         incompressible links of length a.
@@ -11,16 +14,22 @@ class RubberBand:
         Parameters:
             N (int)     The number of links.
             a (float)   The fixed length of links.
+            T (float)   Temperature of the system.
         """
 
         self.N = N
         self.a = a
+        self.T = T
         self.links = self.__sample_links()
 
+    def w(self, f, l):
+        beta = (self.k_B*self.T)**(-1)
+        return np.exp(beta*f*l)
+    
     @property
     def length(self):
         return self.a * np.sum([l.direction for l in self.links])
-
+    
     def __sample_links(self):
         """
         Sample N Link objects with a random direction.
@@ -30,6 +39,4 @@ class RubberBand:
         directions = np.ones(samples.shape)
         directions[samples < 0.5] = -1.
         directions = directions.astype(int)
-        return [Link(d) for d in directions]
-
-
+        return [Link(d) for d in directions]