def format_xyz(title = "Aims Geoemtry")
      output = self.atoms.size.to_s + "\n"
      output << "#{title} \n"
      self.atoms.each{ |a| 
        output << [a.species, a.x.to_s, a.y.to_s, a.z.to_s].join("\t") + "\n"
      }
      output
    end
def format_xyz(title = "Aims Geoemtry")
      output = self.atoms.size.to_s + "\n"
      output << "#{title} \n"
      self.atoms.each{ |a| 
        output << [a.species, a.x.to_s, a.y.to_s, a.z.to_s].join("\t") + "\n"
      }
      output
    end
return a string in xyz format