1. This simulation shows the atomic vibrations for a very long wavelength longitudinal acoustic phonon. Note that all the atoms move in the same direction and the spacing between adjacent atoms is unchanged. In this case, the motions of adjacent atoms are positively correlated and there is no stretching of the bonds for this mode - the change in the width of the pair distribution function σ with temperature is zero. In this and the next two clips, the amplitudes of the atomic vibrations of each atom are the same.