Method of the effective flexible gyroscope in polyatomic molecules

On the basis of the general equations of perturbation theory a procedure is suggested for the partial diagonalization of the vibrational-rotational Hamiltonian of a molecule with respect to the vibrational quantum numbers of a nondegenerate vibrational state. Equations are derived for the complete vibrational-rotational wave function and the "effective" rotational Hamiltonian, which is constructed in such a way that the total energy is represented in the form of a sum of the vibrational and rotational energies.