Stable regime, transition mechanisms

An important area of application for QM methods has been determining and describing reaction pathways, energetics, and transition states for reaction processes between small species. QM-derived first and second derivatives of energy calculated at stable and saddle points on PES can be used under statistical mechanics formulations [33, 34] to yield enthalpies and free energies of structures in order to determine their reactivity. Transition state theory and idealized thermodynamic relationships (e.g., AG[Po—>P] = kTln[P/Po]) allow temperature and pressure regimes to be spanned when addressing simple gas phase and gas-surface interactions. 

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