= Configurational entropy
{title2=$S_{\rm conf}=k_B\ln\Omega_{\rm conf}$}
The part of <entropy> associated with the number of distinct spatial or occupancy arrangements consistent with chosen macroscopic constraints. For equally weighted configurations, the <Boltzmann entropy> formula gives the displayed expression. Unequal probabilities require $-k_B\sum_i p_i\ln p_i$ over configurations. For $n$ indistinguishable empty sites among $M$ otherwise occupied sites, $\Omega_{\rm conf}=\binom{M}{n}$. Vibrational or internal degrees of freedom can contribute additional entropy even at fixed site arrangement.
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