= Adsorption-site grand partition function
{title2=$\Xi_{\rm site}=1+z_s e^{\beta(\mu+\epsilon)}$}
An independent site has one empty state and an occupied set with <binding energy> $-\epsilon$ and vibrational <partition function> $z_s$. Its occupied probability is $\theta=z_s e^{\beta(\mu+\epsilon)}/[1+z_s e^{\beta(\mu+\epsilon)}]$. The denominator enforces the one-molecule-per-site constraint even though the vapour may obey classical statistics. Multiple occupancy or lateral interactions require a different model.
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