Rigid rotor

In rotordynamics, the rigid rotor is a mechanical model of rotating systems. An arbitrary rigid rotor is a 3-dimensional rigid object, such as a top. To orient such an object in space requires three angles, known as Euler angles (ψ, θ, φ). A special rigid rotor is the linear rotor requiring only two angles to describe, for example a diatomic molecule such as HI, HCl, CO. More general molecules are 3-dimensional, such as water (asymmetric rotor), ammonia (symmetric rotor), or methane (spherical rotor).