code: def _export_dx(self, filename, type=None, typequote='"', **kwargs):
          
          root, ext = os.path.splitext(filename)
          filename = root + '.dx'
  
          comments = [
              'OpenDX density file written by gridDataFormats.Grid.export()',
              'File format: http://opendx.sdsc.edu/docs/html/pages/usrgu068.htm#HDREDF',
              'Data are embedded in the header and tied to the grid positions.',
              'Data is written in C array order: In grid[x,y,z] the axis z is fastest',
              'varying, then y, then finally x, i.e. z is the innermost loop.'
          ]
  
          # write metadata in comments section
          if self.metadata:
              comments.append('Meta data stored with the python Grid object:')
          for k in self.metadata:
              comments.append('   ' + str(k) + ' = ' + str(self.metadata[k]))
          comments.append(
              '(Note: the VMD dx-reader chokes on comments below this line)')
  
          components = dict(
              positions=OpenDX.gridpositions(1, self.grid.shape, self.origin,
                                             self.delta),
              connections=OpenDX.gridconnections(2, self.grid.shape),
              data=OpenDX.array(3, self.grid, type=type, typequote=typequote),
          )
          dx = OpenDX.field('density', components=components, comments=comments)
          dx.write(filename)
docs: Export the density grid to an OpenDX file.
  
          The file format is the simplest regular grid array and it is
          also understood by VMD's and Chimera's DX reader; PyMOL
          requires the dx `type` to be set to "double".
  
          For the file format see
          http://opendx.sdsc.edu/docs/html/pages/usrgu068.htm#HDREDF