Pauling went on to compute the configurational entropy in the following way: consider one mole of ice, consisting of N O and 2 N protons. Each O–O bond has two positions for a proton, leading to 2 possible configurations. However, among the 16 possible configurations associated with each oxygen, only 6 are energetically favorable, maintaining the H 2 O molecule constraint. Then an upper bound of the numbers that the ground state can take is estimated as Ω < 2 (⁠ 6 / 16 ⁠). Correspondingly the configurational entropy S 0 = k B ln(Ω) = Nk B ln(⁠ 3 / 2 ⁠) = 0.81 cal/(K·mol) = 3.4 J/(mol·K) is in amazing agreement with the missing entropy measured by Giauque and Stout.