Consider the forces between two slowly moving atoms or molecules in a collision. According to the Born-Oppenheimer approximation, the motion of the nuclei can be separated from the motion of the electrons. In order to describe the forces, defined by the states of the valence electrons, the states have to be given in an appropriate representation: adiabatic or a diabatic.

The motion of a diatomic system is described by the Schroedinger equation. This yields the formal set of exact coupled equations for the nuclear wave functions given in equation 0.11.

The Born-Oppenheimer approximation allows for a substantial simplification of the equations of motion of a molecular system. The Born-Oppenheimer approximation is probably the most powerful tool to describe forces acting between two atoms. This approximation, which is also known as the adiabatic approximation, is based on the enormous mass difference between light and therefore very mobile, electrons, and the heavy slow nuclei. The
ratio *M*/*m* of nuclear to electronic mass runs from a minimum value of 2000 for H to 30,000 for atmospheric gasses and over 400.000 for the heaviest atoms.

The physical consequence of that large mass ratio, is that, to first order, the motion of the nuclei does not influence the highly quantized motion of the electrons. In other words, the interactions between the atoms are described by potentials, which actually are the expectation values of the energy of electronic states. The potentials, as well as the wave functions of the electrons, depend parametrically upon the internuclear separation of the two atoms.

The Born-Oppenheimer approximation can be expected to be valid in the region of nuclear velocity below
,
if the following assumptions hold: firstly the rotational velocity is not much larger than the radial, such that we can concentrate on the radial component of ,
and secondly the electronic states do no vary greatly over distance much smaller than the atomic unit of distance *a*_{0}, i.e.

The mathematical counterpart of the qualitative difference between the electronic motion and the nuclear motion is the recognition that under all reasonable conditions the

In the adiabatic representation, or more precisely the stationary adiabatic representation, of the electronic states, the electronic functions
of the system are defined as the stationary eigenvalues of the electronic Hamiltonian *H*_{e}:

A further interesting property of the stationary adiabatic representation is the famous *non-crossing rule* of von Neumann and Wigner
, which states that two potential curves,
*V*_{ii}^{ad}(*R*) and
*V*_{jj}^{ad}(*R*) may not cross if they have the same symmetry (spin, parity, angular momentum).

The diabatic representation is used to describe two-state and many-state problems: the electronic state is described as either covalent or ionic, with the corresponding electronic wave functions
and
.
In this diabatic representation, electronic states of the same symmetry can violate the non-crossing rule, because they are not the stationary eigenvalues which diagonalise *H*_{e}

= | |||

= |

The probability of the transition from one state to the other is given by the Landau-Zener formula
. The probability is only appreciable in the neighbourhood of the point where the potential energy curves *V*_{cov} and *V*_{ion} cross and for larger velocities.