Reduced Hamiltonian up to the Eighth Order for the CH₃OH Molecule

The reduction of the torsion-rotational Hamiltonian for CH₃OH-type molecules into an effective Hamiltonian including only experimentally determinable parameters is discussed. Reduced Hamiltonians of the fourth, sixth, and eighth orders are given, which include 22, 50, and 95 parameters, respectively A rule is given to write the reduced Hamiltonians higher than the eighth order. In the fourth-order reduced Hamiltonian, it is shown that parameters generally used by previous researchers cannot be determined independently; the k₆{P_γP_a, cos 3γ} term should be replaced by k′₆{P_γP_b, cos 3γ} when k₇ and V₆ are included.