code: def get_okeeffe_distance_prediction(el1, el2):
      
      el1_okeeffe_params = get_okeeffe_params(el1)
      el2_okeeffe_params = get_okeeffe_params(el2)
  
      r1 = el1_okeeffe_params['r']
      r2 = el2_okeeffe_params['r']
      c1 = el1_okeeffe_params['c']
      c2 = el2_okeeffe_params['c']
  
      return r1 + r2 - r1 * r2 * pow(
          sqrt(c1) - sqrt(c2), 2) / (c1 * r1 + c2 * r2)
docs: Returns an estimate of the bond valence parameter (bond length) using
      the derived parameters from 'Atoms Sizes and Bond Lengths in Molecules
      and Crystals' (O'Keeffe & Brese, 1991). The estimate is based on two
      experimental parameters: r and c. The value for r  is based off radius,
      while c is (usually) the Allred-Rochow electronegativity. Values used
      are *not* generated from pymatgen, and are found in
      'okeeffe_params.json'.
  
      Args:
          el1, el2 (Element): two Element objects
      Returns:
          a float value of the predicted bond length