Energy reference in molecular partition functions
= Energy reference in molecular partition functions
{title2=$q_{\rm abs}=e^{-\beta\varepsilon_0}q$}
If energies are measured upward from a species reference $\varepsilon_0$, its absolute single-particle sum is $q_{\rm abs}=e^{-\beta\varepsilon_0}q$. The ideal-gas <chemical potential> is then $\varepsilon_0-k_BT\ln(q/N)$. A zero-point factor can be included either in $q$ or in the reference energy, but not both.