Multireferenzmethoden

Multireferenzmethoden (multireference methods) are computational techniques used in quantum chemistry to accurately describe electronic structures of molecules and atoms, especially those with complex, correlated electron interactions. Unlike single-reference methods, which rely on a single wavefunction approximation based on a dominant electronic configuration, multireference methods consider multiple electronic configurations simultaneously. This approach allows for a more accurate representation of systems where electron correlation effects are significant, such as transition states, diradicals, and excited states.

The core idea of multireference methods is to generate a reference wavefunction incorporating several key configurations, which serve as a basis for further correlation treatments. Common techniques include Complete Active Space Self-Consistent Field (CASSCF) and multireference configuration interaction (MRCI). CASSCF involves selecting an active space of orbitals and distributing electrons among them to optimize the wavefunction, capturing static correlation. MRCI builds upon this by including excitations out of the active space to account for dynamic correlation.

Multireference methods are computationally more intensive than single-reference approaches, such as Hartree-Fock or perturbation theories, due to the complexity of managing multiple configurations. They are particularly valuable in describing bond-breaking processes, excited electronic states, and systems with near-degenerate orbitals. However, their computational cost limits their use to relatively small molecules or offers approximations for larger systems.

Overall, multireference methods are essential tools in theoretical chemistry for accurately modeling complex electronic phenomena, providing insights that are crucial for understanding chemical reactivity, spectroscopy, and material properties.