# Thermal expansion

1. Nov 3, 2013

### aaaa202

In the harmonic approximation, why is the volume of a crystal not temperature dependent? Does it have something to do with the fact that the amplitude of a harmonic oscillator is independent of the frequency?

2. Nov 3, 2013

### DrDu

No, but the mean deviation from the equilibrium position $\langle q \rangle_T=0$ in the harmonic approximation. So the volume won't change on the mean.