N is the total number of data used for fitting, and rmodel and rexp are the calculated and experimental values of the reaction rate, respectively. rmodel is a function according to 𝑃 𝐻 2 , 𝑃 𝑁 2 , and 𝑃 𝑁𝐻 3 with 𝐾 𝑖 , 𝑘 𝑖 as constants (see eq.