# Why does $H_2$ have $C_V$=$7/2 R$ at high temperatures, while the total number of degrees of freedom is 6?

The two hydrogen atoms have 6 degrees of freedom in total. Of them, $3$ contribute to translation, $2$contribute to rotation and $1$ contribute to the vibration.

I know that the vibrations motion is frozen at low temperature due to quantum mechanical effects.

However, then the $C_V$ at high temperature should be $6/2 R$, while experimentally, it is $7/2 R$ (source: Principles of Physics by Walker, Resnick and Halliday)

Edit: The answers reveal that the missing part of specific heat is due to potential energy of vibration. So I am extending the question for clarification. $CO_2$ has total 9 degrees of freedom, of which 3 are translational, 2 are rotational, 4 are vibrational. So, at high temperature, will the $C_V$ of $CO_2$ be $\frac{R}{2} × [3+2+4+4]$? The two 4s are due to kinetic and potential energy of vibrational motion.