code: def center_fit(self, **kwargs):
          
          kwargs.setdefault('s', self.tpr)
          kwargs.setdefault('n', self.ndx)
          kwargs['f'] = self.xtc
          kwargs.setdefault('o', self.outfile(self.infix_filename(None, self.xtc, '_centfit', 'xtc')))
          force = kwargs.pop('force', self.force)
  
          logger.info("Centering and fitting trajectory {f!r}...".format(**kwargs))
          with utilities.in_dir(self.dirname):
              if not self.check_file_exists(kwargs['o'], resolve="indicate", force=force):
                  trj_fitandcenter(**kwargs)
              logger.info("Centered and fit trajectory: {o!r}.".format(**kwargs))
          return {'tpr': self.rp(kwargs['s']), 'xtc': self.rp(kwargs['o'])}
docs: Write compact xtc that is fitted to the tpr reference structure.
  
          See :func:`gromacs.cbook.trj_fitandcenter` for details and
          description of *kwargs* (including *input*, *input1*, *n* and
          *n1* for how to supply custom index groups). The most important ones are listed
          here but in most cases the defaults should work.
  
          :Keywords:
             *s*
               Input structure (typically the default tpr file but can be set to
               some other file with a different conformation for fitting)
             *n*
               Alternative index file.
             *o*
               Name of the output trajectory.
             *xy* : Boolean
               If ``True`` then only fit in xy-plane (useful for a membrane normal
               to z). The default is ``False``.
             *force*
               - ``True``: overwrite existing trajectories
               - ``False``: throw a IOError exception
               - ``None``: skip existing and log a warning [default]
  
          :Returns:
                dictionary with keys *tpr*, *xtc*, which are the names of the
                the new files