Knowledge Bank

A reduced Hamiltonian (AS) was $developed1$ to fit rotational energy levels for asymmetric tops; however, this Hamiltonian could break down for near-symmetric tops. Two methods of overcoming this difficulty are to change to a reduced Hamiltonian (NS) that is valid in the symmetric top $limit2$ or to change representations; these methods are discussed in more detail, with applications to the spectra of several molecules. Under certain conditions the usual technique for assignment of the quantum labels $JKaKc$ to energy levels can be done incorrectly. Several resolutions of this problem, including one that speeds greatly the calculation of molecular constants for certain near-symmetric top molecules, are presented.