code: def partial_transform(self, traj):
          
          d = md.geometry.compute_distances(traj, self.pair_indices,
                                            periodic=self.periodic)
          return d ** self.exponent
docs: Featurize an MD trajectory into a vector space via pairwise
          atom-atom distances
  
          Parameters
          ----------
          traj : mdtraj.Trajectory
              A molecular dynamics trajectory to featurize.
  
          Returns
          -------
          features : np.ndarray, dtype=float, shape=(n_samples, n_features)
              A featurized trajectory is a 2D array of shape
              `(length_of_trajectory x n_features)` where each `features[i]`
              vector is computed by applying the featurization function
              to the `i`th snapshot of the input trajectory.
  
          See Also
          --------
          transform : simultaneously featurize a collection of MD trajectories