Convergence of the reduced Hamiltonian with centrifugal distortion in asymmetric top molecules

It is shown that the goodness of fit of observed spectra of asymmetric top molecules, when using the reduced Hamiltonian of Watson, can be affected by the choice of molecular z-axis. The spectra of carbonyl fluoride and sulphur dioxide are used to illustrate this effect, and the convergence properties of the Hamiltonian are related to the order of magnitude of the co-efficient s₁₁₁ used in the contact transformation.