Sackur–Tetrode Equation

$\sigma =N\left(\ln \frac{n_Q}{n}+\frac{d}{2}+1\right)$

• $\sigma$ is the fundamental entropy
• For ideal gases in 3D
• $n$ is the density of particles per unit volume
• $n_Q$ is the quantum concentration
• $d$ is the number of spatial dimensions