An algebraic scheme for describing the bending dynamics of tetratomic molecules including linear, bent planar, and bent a-planar species is introduced. The correlation diagram linear-cis-bent and linear-transbent is constructed. Effective potential energy functions are evaluated by exploiting the method of coherent states. A sample calculation of the bending vibrations of C 2H 2 in its X 1? g + electronic ground state is performed. © 2009 American Chemical Society.