*2.2. Molar Density*

For pressures below one atmosphere, the molar density can be calculated by assessing the refractive index and using the extended Lorentz–Lorenz equation

$$\varphi = \frac{2}{3A\_R}(n-1)[1 + b\_{n-1}(n-1)],\tag{3}$$

where *AR* and *bn*−<sup>1</sup> are the molar dynamic polarizability [**? ?** ]. The latter is given by −(<sup>1</sup> + <sup>4</sup>*BR*/*A*<sup>2</sup> *<sup>R</sup>*)/6, where, in turn, *BR* is the second refractivity virial coefficient in the Lorentz–Lorenz Equation [**???** ].

## *2.3. Pressure*

The molar density can then be used to assess the pressure as

$$P = RT\rho[1 + B\_{\rho}(T)\rho],\tag{4}$$

where *R* is the ideal gas constant, *T* is the temperature of the gas, and *Bρ*(*T*) is the second density virial coefficient.

For more detailed theoretical descriptions of the Lorentz–Lorenz equation and the equation of state, and for expressions valid for higher pressures, the reader is referred to the literature, e.g., [**????????** ].
