Diabatic Hamiltonian weakness, adiabatic coupling

The Adiabatic Approximation [51] Dealing With the Strong Anharmonic Coupling Theory of Weak H-Bonds Partition of the Full Hamiltonian into Diabatic and Adiabatic Parts Weakness of the Diabatic Hamiltonian [122]... 

Now, recall that for weak hydrogen bonds the high-frequency mode is much faster than the slow mode because 0 m 20 00. As a consequence, the quantum adiabatic approximation may be assumed to be verified when the anharmonic coupling parameter aG is not too strong. Thus, neglecting the diabatic part of the Hamiltonian (22) and using Eqs. (18) to (20), one obtains... 

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