Figure 1.

A three-state VB system with E₁, E₂, and E₃ and the corresponding diabatic free energies g₁, g₂, and g₃ can be projected onto a two-state VB representation. The resulting surfaces E′₁ and E′₂ and free energies g′₁ and g′₂ no longer represent pure resonance structures, but their mixing results in the same ground-state potential surface E_g (with a corresponding free energy surface ḡ). When the donor and the acceptor are held at a distance R, the stabilization resulting from the mixing of the two unpure states is given by H₁₂(R,r), and its value at the barrier (r = r′) is denoted simply by H₁₂. The coordinate r corresponds to proton movement at fixed R.