Additive-volume two-level system
= Additive-volume two-level system
A model in which each excitation changes <volume> by a fixed increment $v$. If the all-ground configuration has <volume> $V_0$, then $V=V_0+v\langle N_{\rm ex}\rangle$. Energies and <statistical weights> must be specified under the relevant mechanical constraint; <volume> endpoints alone do not define arbitrary intermediate configurations.