mechanics describes a mechanical
system as a set of particles (which in a limiting
case can form continuous media) having a well-defined geometry at any given
time, and undergoing motions determined by applied forces and by the initial
positions and velocities of the particles. The forces themselves may have
electromagnetic or quantum
origins. Classical statistical mechanics uses the same
physical model, but treats the geometry and velocities as uncertain,
statistical quantities subject to random thermally-induced fluctuations.
Classical mechanics and classical statistical mechanics give a good
account of many mechanical
properties and behaviors of molecules;
but for describing the electronic
properties and behaviors of molecules,
they are often useless.