Coordinate systems for describing the orientational configuration of dimers. One of the monomers is fixed at the origin of the reference coordinates such that the center of mass (identified by filled circles) is located at the origin, and the principal axes of inertia align with the coordinate axes, x, y, and z. The polar coordinates of the center of mass of the second monomer are given by θ and φ; the orientation of the second monomer is described by the Euler angles that define the rotation between the reference coordinates and the fixed body coordinates, (x′, y′, z′). The latter are defined by the principal axes of interia of the second monomer.