MøllerPlessetPerturbationstheorie

MøllerPlessetPerturbationstheorie, commonly abbreviated MP perturbation theory, is a post-Hartree–Fock method in quantum chemistry that treats electron correlation by applying Rayleigh–Schrödinger perturbation theory to a Hartree–Fock reference wavefunction. Starting from the electronic Hamiltonian for fixed nuclei, one partitions the Hamiltonian into a zeroth-order Fock operator and a perturbation given by the fluctuation potential. Energy corrections E^(n) are computed, with E^(1)=0 for the MP scheme, so the leading correction is E^(2), called the MP2 correlation energy. Higher orders, MP3, MP4, etc., can be defined, but become progressively more expensive and less reliable; MP2 is by far the most widely used.

MP2 is reasonably accurate for many closed-shell systems and simple valence bonds and is size-extensive, but it can overbind dispersion forces and fail for bond dissociation or near-degeneracy situations. The method is non-variational and can be sensitive to the choice of basis set; results may deteriorate for systems with strong static correlation.

Extensions include MPn based on a single determinant reference and various multireference MBPT approaches for near-degenerate situations. In modern practice MP, especially MP2, has largely been superseded by coupled-cluster methods (e.g., CCSD, CCSD(T)) for higher accuracy, though MP2 remains a useful, computationally affordable correction in many calculations.