= Fugacity
{disambiguate=statistical mechanics}
{title2=$z=e^{\beta\mu}$}
= Quantum gas fugacity
{synonym}
The dimensionless <quantum gas fugacity> weights <particle number> in the <grand canonical ensemble>: a state with <energy> $E$ and <particle number> $N$ has weight $z^Ne^{-\beta E}$. The <chemical potential> and one-particle <energies> must use the same reference. When the lowest one-particle <energy> is zero, $z\ll1$ makes both <fermion> and <boson> occupations small throughout the low-energy spectrum. This dimensionless quantity must be distinguished from the pressure-valued fugacity used in chemical thermodynamics.
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