  The coupling of electronic and vibrational motion is studied by two canonical
transformations namely normal coordinate transformation and momentum
transformation on molecular Hamiltonian. It is shown that by these
transformations we can pass from crude approximation to adiabatic approximation
and then to non-adiabatic (diabatic) Hamiltonian. This leads to renormalized
fermions and renormalized diabatic phonons. Simple calculations on $H_{2}$,
$HD$, and $D_{2}$ systems are performed and compared with previous approaches.
Problem of electronic quasi-degeneracy is discussed.
