code: public double getBondLength() {
          PharmacophoreAtom atom1 = PharmacophoreAtom.get(getAtom(0));
          PharmacophoreAtom atom2 = PharmacophoreAtom.get(getAtom(1));
          return atom1.getPoint3d().distance(atom2.getPoint3d());
      }
docs: Get the distance between the two pharmacophore groups that make up the constraint.
  
  @return The distance between the two groups