def format_geometry_in
      output = ""
      if self.lattice_vectors
        output << self.lattice_vectors.collect{|v| "lattice_vector #{v[0]} #{v[1]} #{v[2]}"}.join("\n")
        output << "\n"
      end
      output << self.atoms.collect{|a| a.format_geometry_in}.join("\n")

      output
    end
def format_geometry_in
      output = ""
      if self.lattice_vectors
        output << self.lattice_vectors.collect{|v| "lattice_vector #{v[0]} #{v[1]} #{v[2]}"}.join("\n")
        output << "\n"
      end
      output << self.atoms.collect{|a| a.format_geometry_in}.join("\n")

      output
    end
Return a string formatted in the Aims geometry.in format.