The molar mass formula_195 (or molecular mass) is normally not included in the EOS formula, but it usually enters the characterization of the EOS parameters. From the equation of state the molar volume of the reservoir fluid (mixture) is calculated. The molar volume formula_197 is converted to mole density formula_198 (also called mole concentration and denoted formula_199), and then scaled to be reduced mole density formula_200. The correlation for dilute gas viscosity of a mixture is taken from Herning and Zipperer  (1936) and is  The correlation for dilute gas viscosity of the individual components is taken from  Stiel and Thodos (1961) and is  where  The principle of corresponding states (CS principle or CSP) was first formulated by van der Waals, and it says that two fluids (subscript a and z) of a group (e.g. fluids of non-polar molecules) have approximately the same reduced molar volume (or reduced compressibility factor) when compared at the same reduced temperature and reduced pressure. In mathematical terms this is  When the common CS principle above is applied to viscosity, it reads  Note that the CS principle was originally formulated for equilibrium states, but it is now applied on a transport property - viscosity, and this tells us that another CS formula may be needed for viscosity. In order to increase the calculation speed for viscosity calculations based on CS theory, which is important in e.g. compositional reservoir simulations, while keeping the accuracy of the CS method, Pedersen et al.  (1984, 1987, 1989) proposed a CS method that uses a simple (or conventional) CS formula when calculating the reduced mass density that is used in the rotational coupling constants (displayed in the sections below), and a more complex CS formula, involving the rotational coupling constants, elsewhere. The simple corresponding state principle is extended by including a rotational coupling coefficient formula_208 as suggested by Tham and Gubbins  (1970). The reference fluid is methane, and it is given the subscript z. The interaction terms for critical temperature and critical volume are  The parameter formula_124 is usually uncertain or not available. One therefore wants to avoid this parameter. Replacing formula_133 with the generic average parameter formula_216 for all components, gives  The above expression for formula_220 is now inserted into the equation for formula_221. This gives the following mixing rule  Mixing rule for the critical pressure of the mixture is established in a similar way.