Parameters in the compound energy formalism for ionic systems

The compound energy formalism, CEF, involves many model parameters. They are evaluated to give the best fit to the experimental information. The optimisation is simpler if less parameters need to be adjusted. The maximum number of independent parameters that can be evaluated depends on the information available. The best choice of parameters is first discussed for simple ionic substances with an internal variable, then for solutions of two or four such substances.To reduce the number of parameters, independent parameters are conveniently defined as combinations of primary model parameters. That may be possible when there is an internal variable, which can take only one value, the value that minimizes the Gibbs energy. Such combinations may be regarded as the true optimisation parameters and they may be used actively during an optimisation. The present discussion deals with substances with an internal variable and mixtures, which may have more than one internal variable.The conclusions apply equally well to non-ionic systems if the information is limited to stoichiometric compositions. The optimisation parameters should then be defined for stoichiometric overall compositions.