code: def insert(self, i, species, coords, validate_proximity=False,
                 properties=None):
          
          new_site = Site(species, coords, properties=properties)
          if validate_proximity:
              for site in self:
                  if site.distance(new_site) < self.DISTANCE_TOLERANCE:
                      raise ValueError("New site is too close to an existing "
                                       "site!")
          self._sites.insert(i, new_site)
docs: Insert a site to the molecule.
  
          Args:
              i (int): Index to insert site
              species: species of inserted site
              coords (3x1 array): coordinates of inserted site
              validate_proximity (bool): Whether to check if inserted site is
                  too close to an existing site. Defaults to True.
              properties (dict): Dict of properties for the Site.
  
          Returns:
              New molecule with inserted site.