An angular singularity-free method for the dynamics of the rovibrational motion of diatomic molecules

An angular singularity-free method for the dynamics of the rovibrational motion of diatomic molecules has been proposed. The present method is based on a formulation that the orientation of a diatomic molecule is expressed by the Cartesian coordinates of the unit vector parallel to the molecular axis. In this light, the present method can be considered as a modification of a method proposed for the rigid rotational motion to the rovibrational motion. Illustrations of the present method have been performed for the classical dynamics of the rovibrational motion of a diatomic molecule, and for the semiclassical dynamics of the collision between an atom and a diatomic molecule, including comparisons with other conventional methods. It is concluded that the present method is free from angular singularity, and that the present method is superior to other conventional methods in view of computational efficiency.