Fig. 6. (a) Free energy diagram for the formation of [Ni-1–H]^– (black line) and [H–Ni-2]^– (red line) intermediates during the HER process. Free-energy values are given in kcal mol^–1. The geometric conformation of [Ni-1–H]^– (b) and [H–Ni-2]^– (c) active intermediates during the HER process.