In section 1, the concept of anensemble(of molecules) in the statistical mechanics is shortly introduced. The following three sections (sections 2–4) serve as a summary of the energy levels in the molecules and the correspondingdensity of statesandpartition functions, which are components in statistical calculations. The last two sections summarize the framework of practical statistical calculations.

## Outlines

### Section 1: «Introduction – Statistical Mechanics»

Two typical

*ensembles*of molecules,*canonical ensemble*and*microcanonical ensemble*are introduced in this section, and their application to the rate theories will be discussed.### Section 2: «Electronic States of Atoms and Molecules»

Chemical reactions often involve the changes of
electronic states, and thus, the degeneracy (or the statistical weight)
changes from the reactants, via transition states (and/or the
intermediates), to the products. A concise summary for proper counting
of the degeneracy of elecronic states will be shown.

### Section 3: «External Nuclear Motions»

In this section, external motions of nuclei
in molecules, traslational motion and (external) rotation, are
described and their quantum mechanical eigenstates, density of
states, and partition function are formulated. Also, a brief summary
is given for the rotational symmetry number and number of (optical)
isomers.

### Section 4: «Intramolecular Motions»

Vibrational motion and internal rotaion
are described with their density of states and partition function.
A concise summary of the treatment for free/hindered internal rotation
is also given.

### Section 5: «Canonical Statistics»

In this section, application of canonical
statistics for thermodynamic functions and thermal rate theory
are discribed.

### Section 6: «Microcanonical Statistics»

Microcanonical variation of the rate theory,
RRKM or microscopic rate constant, and its application to the
unimolecular reactions will be discussed.

## Suggestions for Further Reading

For basic subjects including thermodynamics, statistical thermodynamics,
quantum mechanics of electronic states and nuclear motions, and

*etc*.;- P. Atkins and J. de Paula,
*Atkins' Physical Chemistry, 7th Ed.*, Oxford University Press (2002).

For trasitional statistical mechanics of gas-phase molecules;

- J. H. Knox,
*Molecular thermodynamics - An Introduction to Statistical Mechanics for Chemists*, John Wiley & Sons (1971). - D. A. McQuarrie,
*Statistical Mechanics*, University Science Books (2000).