Spectroscopy in the Presence of Geometrical Constraints: A Torsional   Pendulum
  We demonstrate that an effect other than anharmonicity can severely distort the spectroscopic signatures of quantum mechanical systems. This is done through an analytic calculation of the spectroscopic response of a simple system, a charged torsional pendulum. One may look for these effects in the optical data of real systems when for example a significant rocking component of rigid polyhedra plays a significant role in the lattice dynamics. 
Jason N. Hancock, Trieu T. Mai, Zack Schlesinger