A general method is presented for the evaluation of multipole-multipole force constants, and the temperature dependence is calculated by representing each H₃ triangle as a superposition of a ring of charge q_r and a triangle of charge q_t, with q_t increasing and q_r decreasing as the temperature falls through T_c. Changes in these multipole forces with decreasing temperature depress the initial gradients of the transverse acoustic modes by coupling with the librational doublet, with the latter exhibiting soft-mode behaviour. The calculations also predict that the stability of the structure is very dependent on the magnitude of the interaction between clusters, in agreement with theory, and that the chloride structure is more stable and has a higher transition temperature than the iodide, both of which are in agreement with experiment.