docs: def convert_formula_to_atomic_fractions(formula):
      """
      Converts a chemical formula to an atomic fraction :class:`dict`.
  
      Args:
          formula (str): chemical formula, like Al2O3. No wildcard are accepted.
      """
      mole_fractions = {}
      total_mole_fraction = 0.0
  
      for match in CHEMICAL_FORMULA_PATTERN.finditer(formula):
          symbol, mole_fraction = match.groups()
  
          z = pyxray.element_atomic_number(symbol.strip())
  
          if mole_fraction == '':
              mole_fraction = 1.0
          mole_fraction = float(mole_fraction)
  
          mole_fraction = float(mole_fraction)
          mole_fractions[z] = mole_fraction
          total_mole_fraction += mole_fraction
  
      # Calculate atomic fractions
      atomic_fractions = {}
      for z, mole_fraction in mole_fractions.items():
          atomic_fractions[z] = mole_fraction / total_mole_fraction
  
      return atomic_fractions