A reference to the external file containing the structural models 
      in the ensemble. The number of models in the external file should
      correspond to the number of models in the ensemble. This data item 
      is a pointer to _ihm_external_files.id in the IHM_EXTERNAL_FILES 
      category.
      It is recommended that the large ensemble files be stored as separate 
      zip files within the same DOI. It is also recommended that large sphere 
      model ensembles be in binary format, which facilitates faster access. 
      Currently, a binary dump of co-ordinates in dcd format is suggested. 
      The topology can be inferred from the IHM_SPHERE_OBJ_SITE and the
      ATOM_SITE categories in the corresponding mmCIF file.