BornOppenheimertilnærmingen

BornOppenheimertilnærmingen, often referred to as the Born-Oppenheimer approximation, is a fundamental concept in quantum chemistry and physics that simplifies the description of molecular systems. It is based on the significant difference in mass between atomic nuclei and electrons. Electrons are much lighter and move far more rapidly than the nuclei. This disparity allows for the approximation that the electrons instantaneously adjust their positions in response to the nuclear framework, as if the nuclei were stationary.

The approximation allows researchers to decouple the electronic and nuclear motions within a molecule. In practice, this means that the total molecular wavefunction can be separated into an electronic wavefunction and a nuclear wavefunction. The electronic Schrödinger equation is solved for a fixed nuclear configuration, yielding the electronic energy and wavefunction for that specific arrangement of nuclei. This process is repeated for various nuclear configurations to map out the potential energy surface of the molecule.

The nuclear motion, including vibrations and rotations, is then described as moving on this calculated potential energy surface. This separation greatly simplifies the complex many-body problem of solving the Schrödinger equation for an entire molecule, making calculations tractable. The Born-Oppenheimer approximation is a cornerstone for understanding molecular structure, spectroscopy, and chemical reactions. However, it is an approximation, and its validity breaks down in situations where electronic and nuclear motions are strongly coupled, such as in certain photochemical processes or when dealing with very light nuclei like hydrogen.