Thermodynamic pressure is the logarithm of a statistical-mechanical partition
function divided by the system size. For 
spins with reference measure 
and energy 
, the convention is
 |
Its limit as 
, when it exists, is the infinite-system
pressure. The physical free energy per spin in this convention is 
. For uniform probability
on binary spins, 
at zero interaction, whereas counting
measure gives 
.
In a disordered system, quenched pressure averages 
over the disorder, while annealed pressure uses 
. These operations generally differ.
