code: def to_string(self):
          
          # define coord system, use Cartesian
          output = ["1.0 0.0 0.0",
                    "0.0 1.0 0.0",
                    "0.0 0.0 1.0"]
          # add lattice vectors
          m = self.structure.lattice.matrix
          output.append("{:6f} {:6f} {:6f}".format(*m[0]))
          output.append("{:6f} {:6f} {:6f}".format(*m[1]))
          output.append("{:6f} {:6f} {:6f}".format(*m[2]))
          # add species
          for site in self.structure:
              species_str = []
              for sp, occu in sorted(site.species.items()):
                  if isinstance(sp, Specie):
                      sp = sp.element
                  species_str.append("{}={}".format(sp, occu))
              species_str = ",".join(species_str)
              output.append("{:6f} {:6f} {:6f} {}".format(site.frac_coords[0],
                                                          site.frac_coords[1],
                                                          site.frac_coords[2],
                                                          species_str))
  
          return "\n".join(output)
docs: Returns a structure in mcsqs rndstr.in format.
          :return (str):