def add_cp_top(ax,xlabel='C$_p$ (bohr$^-2$)'):
  """ add Cp as top xlabel when actual xlabel is 1/Cp.
   Cp is the proton orbital gaussian exponent.  """
  ax1 = ax.twiny()
  ax1.set_xticks( ax.get_xticks() )
  ax1.set_xlim( ax.get_xlim() )
  ax1.set_xlabel(xlabel)

  xlabels = []
  for tick in ax1.get_xticks():
    inv = 1./tick
    xlabels.append("%3.1f"%inv)
  # end for tick
  ax1.get_xaxis().set_ticklabels( xlabels )
  return ax1