Accurate Potential Energy Surface for Quartet State HN and Interplay of N() + NH(Σ) versus H + N(Σ) Reactions.

A global potential energy surface for the lowest quartet state of HN is reported for the first time from accurate multireference ab initio calculations extrapolated to the complete basis set limit using the double many-body expansion method. All its stationary points are characterized, with the lowest quartet of HN predicted to have a bent global minimum 36 kcal mol below the N() + NH(Σ) asymptote, from which it is barrierlessly achievable. The entire set of calculated ab initio points has been fitted for energies up to 1000 kcal mol above the global minimum with an RMSD of 0.