ABSTRACT
The forces between atoms are due to the motion and distribution of electrons and must be calculated, therefore, using quantum mechanics. In practice, atoms will all be in their ground electronic and nuclear states. In a diatomic molecule, the authors are now considering an individual with a Hamiltonian that is a function of twice as many variables as before. Indeed, there are now two sets of coordinates and momenta for the two atoms, as well as two masses, etc. The internal partition function consists of the orbiting and vibrating of the reduced mass. The amplitude of the vibration is very small, giving rise to a separation of the motions of rotation and vibration or, in other words, to the rigid rotor-harmonic oscillator approximation. In a chemical reaction, a set of chemical compounds transforms into another, and molecules change their identities.
