Kay's idea was adopted by T. W. Leland, who applied it to the molecular parameters, ϵ ϵ , σ σ , which are related to a , b through T c , p c by a ∝ ∝ ϵ ϵ σ σ 3 and b ∝ ∝ σ σ 3 (see the introduction to this article). Using these together with the quadratic form of a , b for mixtures produces σ σ x 3 = ∑ ∑ i = i n ∑ ∑ j = 1 n σ σ i j 3 x i x j and ϵ ϵ x = [ ∑ ∑ i = 1 n ∑ ∑ j = 1 n ϵ ϵ i j σ σ i j 3 x i x j ] [ ∑ ∑ i = i n ∑ ∑ j = 1 n σ σ i j 3 x i x j ] − − 1 which is the van der Waals approximation expressed in terms of the intermolecular constants. This approximation, when compared with computer simulations for mixtures, are in good agreement over the range 1 / 2 < ( σ σ 11 / σ σ 22 ) 3 < 2 , namely for molecules of not too different diameters. In fact Rowlinson said of this approximation, "It was, and indeed still is, hard to improve on the original van der Waals recipe when expressed in [this] form".