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In section 7.2 of Rief's "Fundamentals of Statistical and Thermal physics". While calculating the partition function for ideal gas he writes:

$$ \begin{array}{l} \displaystyle{Z' = \int{ \exp{\left\{- \beta \left[\frac{1}{2m} \left({\mathbf{p}_1}^2 + \cdots + {\mathbf{p}_N}^2 \right) + U\left(\mathbf{r}_1, \dots , \mathbf{r}_N\right) \right]\right\}}} } \\ \hspace{15em} \displaystyle{\frac{d^3\mathbf{r}_1 \cdots d^3\mathbf{r}_N d^3\mathbf{p}_1 \dots d^3\mathbf{p}_N}{{h_0}^{3N}}} \end{array} $$

Then he says

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Why is $U$ taken to be zero?

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    $\begingroup$ For an ideal gas we take by definition $U=0$. $\endgroup$ – Dani Oct 4 '18 at 7:03
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    $\begingroup$ $U$ is the potential energy as stated in your equation and the text, not internal energy. $\endgroup$ – K_inverse Oct 4 '18 at 7:27
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If a gas is ideal, then there are assumed to be no interactions between the gaseous molecules. This means they can't have any potential energy (they don't interact with anything to give themselves this), and so their only form of energy is kinetic energy, which is the first term in that integral.

Thus for an ideal gas, $U=0$ (where $U$ is the potential energy).

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