In Equation , it was assumed that electronic, rotational, and vibrational motion may be treated separately, given that they occur on different time scales. The translation is not affected by the field and the ideal gas model can be applied: µ trans gas (p, T , F ) = µ trans gas (p, T ). For the electronic energy, density-functional theory already provides a high-quality description: µ elec gas (p, T , F ) ≈ E vac mol (T , F ) As discussed above, the energy depends on the orientation of the molecule due to the anisotropic polarizability. In an ensemble average, the orientations of the TCNE molecules follow a Boltzmann distribution, which results in a temperature dependence of the electronic energy. For the special case of TCNE, this temperature dependence is small enough to be neglected (i.e., µ elec gas (p, T , F ) ≈ E vac mol (F )). We note, however, that for molecules with even more anisotropic polarizabilities, this may not be the case.