  In 1926, E. Schroedinger published a paper solving his new time dependent
wave equation for a displaced ground state in a harmonic oscillator (now called
a coherent state). He showed that the parameters describing the mean position
and mean momentum of the wave packet obey the equations of motion of the
classical oscillator while retaining its width. This was a qualitatively new
kind of correspondence principle, differing from those leading up to quantum
mechanics. Schroedinger surely knew that this correspondence would extend to an
N-dimensional harmonic oscillator. This Schroedinger Correspondence Principle
is an extremely intuitive and powerful way to approach many aspects of harmonic
solids including anharmonic corrections.
