= Constrained order-parameter free energy
{title2=$F_c(M)=-k_BT\,V^{-1}\log Z_M$}
Fix the spatial mean of a scalar <order parameter> to $M$ and sum the <Boltzmann weights> of all configurations with that mean. The resulting constrained <partition function> $Z_M$ defines this <free-energy density>, with an additive normalization fixed consistently. For an applied <conjugate field> $h$, $Z(h)=\int dM\,e^{-\beta_TV[F_c(M)-hM]}$, up to the chosen measure normalization. The <thermodynamic limit> gives $f(h)=\inf_M[F_c(M)-hM]$. A homogeneous <Landau free energy> is an approximation to this constrained potential; phase-separated configurations can convexify it in the full <thermodynamic limit>.
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