Combining rules

The combining rules (also known as mixing rules) for binary mixtures are variously given by

Berthelot rule

${ϵ}_{ij}=\sqrt{{ϵ}_{ii}{ϵ}_{jj}}$

Lorentz rule

${\sigma }_{ij}=\frac{{\sigma }_{ii}+{\sigma }_{jj}}{2}$

See also Lennard-Jones model

Admur and Mason

For the second virial coefficient of a mixture (Ref. 1)

$B_{ij}=\frac{{(B_{ii}^{1/3}+B_{jj}^{1/3})}^3}{8}$

References

1. I. Amdur and E. A. Mason "Properties of Gases at Very High Temperatures", Physics of Fluids 1 pp. 370-383 (1958)