The derivation of the rotational potential function from atom–atom potentials. I. Ammonium–chlorine compounds

The rotational potential function of the ammonium ion in the ammonium hexachlorides and the disordered phases of NH4Cl and α-NH4HgCl3 are derived from atom–atom potentials of the Buckingham (exp-6) type. The parameters in the atom–atom potentials are determined by matching computed and observed values of the librational and tunneling frequencies of NH4Cl, (NH4)2SnCl6, and (NH4)2PdCl6. The procedure presented in this paper is particularly useful in the determination of the potential function of noncubic ammonium compounds, where there are a large number of parameters in the potential function. The volume dependence of the parameters in the potential function is easily computed when atom–atom potentials are used to derive the rotational potential function.