manybodyeffekter

Manybodyeffekter, or many-body effects, refer to phenomena in quantum systems that arise from interactions among many particles and cannot be understood by treating each particle independently. In quantum systems, electron–electron interactions produce correlation and entanglement that go beyond simple mean-field descriptions.

In practice, calculations start from a model of interacting particles and introduce techniques to account for residual interactions. Mean-field theories such as the Hartree–Fock approximation neglect dynamic correlations, while many-body theory includes them through Green's functions, self-energy corrections, and perturbation theory. Diagrammatic methods, Dyson equations, and spectral functions are used to describe excitation spectra and finite lifetimes.

A set of widely used approaches includes the GW approximation for the electronic self-energy and the Bethe–Salpeter equation for excitons, as well as dynamical mean-field theory (DMFT) for strongly correlated materials. For finite or low-dimensional systems, exact diagonalization, density matrix renormalization group (DMRG), and quantum Monte Carlo provide numerically controlled results. In solids, many-body effects renormalize quasiparticle energies, broaden spectral features, and give rise to collective modes such as plasmons.

Common manifestations include correlation energy beyond Hartree–Fock, satellite structures in photoemission, renormalized band gaps, Mott metal–insulator transitions, Kondo phenomena, and unconventional superconductivity. In chemistry and materials science, such effects influence reaction dynamics, optical spectra, and transport properties.

Understanding many-body effects is essential for accurately predicting electronic structure, optical responses, and emergent phenomena in complex quantum systems.