def total_energy
      etot = self.geometry_steps.collect{|gs| gs.total_energy }.compact.last
      if etot.nil? 
        Float::NAN
      else
        etot
      end
    end
def total_energy
      etot = self.geometry_steps.collect{|gs| gs.total_energy }.compact.last
      if etot.nil? 
        Float::NAN
      else
        etot
      end
    end
Returns the best available value of the total energy