We used density functional theory (DFT) to compute the reaction enthalpies ∆Hs of several important chemical reactions investigated in this work. In DFT, wavefunctions were described using plane waves for valence electrons and projected augmented wave potentials for the core electrons, as implemented in the Vienna Ab initio Simulation Package. The unknown exchange and correlation contribution was approximated by the generalized gradient approximation (GGA) by Perdew, Burke, and Ernzerhof (PBE). The parameters used for geometry optimization and energy calculations follow the MITRelaxSet, as in Ref. 64. We used a plane wave energy cut-off of 520 eV, and the total energy was converged to 10 -5 eV per cell. Geometries (coordinates, volumes, and cell shapes) were considered converged when the total energy difference between two consecutive geometry steps was lower than 10 -4 eV (per cell).