**Born Oppenheimer approximation** In the Born Oppenheimer (BO) approximation only the movements of electrons are considered and the atoms are considered to be in a rigid, fixed position.

It is assumed in the BO approximation that the movement of nucleii due to the movements of electrons can be neglected. Nucleii are much heavier than electrons. It is assumed that the molecular wavefunction can be approximated by the product of a electronic and a nuclear wavefunction.

The BO approximation is applicable if the potential energy surfaces of different states of the molecule are well separated. In regions where the potential energy curves of different states overlap the BO approximation is inapplicable.

The Born-Oppenheimer is sometimes also called the adiabatic approximation.

__Related topics:__ Condon Approximation

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1341844204/0#0 Conical intersection

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1289733283/0#0 PES: Potential energy surface

http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1216671925/0#0 __References:__ European Summerschool in Quantum Chemistry 2013, Book I, p201