Help with Physical Chemistry/Quantum Mechanics

"Briefly and clearly define each of the following and explain the significance. If you use an equation in your answer, define in words each symbol you use."

a) Separability approximation(s) to describe the molecular dynamics and spectra.

b)Harmonic (anharmonic) oscillator/rigid(non-rigid) models to describe molecular dynamics and spectra

c) Pure rotational spectra of a symmetric (linear) molecule

d) Ensemble average and its relationship to the thermodynamic quantity

e) Mean energy, heat capacity and pressure and their relationship to the partition function