  We theoretically demonstrate that at a finite temperature, an effective wall
thickness of a single walled carbon nanotube (SWNT) should be $W=W_s+W_d$,
where $W_s$ is the static thickness defined as the extension of the outmost
electronic orbit and $W_d$ the dynamic thickness due to thermal vibration of
atoms. Both molecular simulations and a theoretical analysis show that $W_d$ is
proportional to $\sqrt{T}$. We find that the increase of dynamic thickness with
temperature is the main mechanism of Raman spectrum shift. The introduction of
dynamic thickness changes some conclusions about the Young's modulus and
reduces the values of thermal conductivity.
