code: def setup_hydrocarbon_ff(graph):
      
      # A) Define parameters.
      # the bond parameters:
      bond_params = {
          (6, 1): 310*kcalmol/angstrom**2,
          (6, 6): 220*kcalmol/angstrom**2,
      }
      # for every (a, b), also add (b, a)
      for key, val in list(bond_params.items()):
          if key[0] != key[1]:
              bond_params[(key[1], key[0])] = val
      # the bend parameters
      bend_params = {
          (1, 6, 1): 35*kcalmol/rad**2,
          (1, 6, 6): 30*kcalmol/rad**2,
          (6, 6, 6): 60*kcalmol/rad**2,
      }
      # for every (a, b, c), also add (c, b, a)
      for key, val in list(bend_params.items()):
          if key[0] != key[2]:
              bend_params[(key[2], key[1], key[0])] = val
  
      # B) detect all internal coordinates and corresponding energy terms.
      terms = []
      # bonds
      for i0, i1 in graph.edges:
          K = bond_params[(graph.numbers[i0], graph.numbers[i1])]
          terms.append(BondStretchTerm(K, i0, i1))
      # bends (see b_bending_angles.py for the explanation)
      for i1 in range(graph.num_vertices):
          n = list(graph.neighbors[i1])
          for index, i0 in enumerate(n):
              for i2 in n[:index]:
                  K = bend_params[(graph.numbers[i0], graph.numbers[i1], graph.numbers[i2])]
                  terms.append(BendAngleTerm(K, i0, i1, i2))
  
      # C) Create and return the force field
      return ForceField(terms)
docs: Create a simple ForceField object for hydrocarbons based on the graph.