The parameter ξ ξ 12 is an additional state-independent interaction parameter for the mixture. The parameter η η 12 is usually set to unity since the arithmetic mean can be considered physically plausible for the cross-interaction size parameter. The parameter ξ ξ 12 on the other hand is often used to adjust the geometric mean so as to reproduce the phase behavior of the model mixture. For analytical models, e.g. equations of state, the deviation parameter is usually written as k 12 = 1 − − ξ ξ 12 . For ξ ξ 12 > 1 , the cross-interaction dispersion energy and accordingly the attractive force between unlike particles is intensified, and the attractive forces between unlike particles are diminished for ξ ξ 12 < 1 .