The van Laar equation is an old correlation which is still frequently used to compute liquid activity coefficients. It can be derived by discarding ternary and higher terms in Wohl's equation (an alternate expansion of the excess Gibb's energy).
Two parameters are required for each binary pair. There is no facility to make the parameters temperature dependent. Because of its functional form, it cannot represent maxima or minima in the relationship between activity coefficient and mole fraction.
See "van Laar Equation" under "Liquid Activity Coefficient Methods" in the PRO/II Reference Manual, Volume I, Chapter 2, for more information on this method.