postHartreeFocktekniker

PostHartreeFocktekniker refers to the class of quantum chemical methods that go beyond the Hartree–Fock approximation by including electron correlation explicitly in the many-electron wavefunction. These techniques aim to improve predictions of molecular energies, geometries, and properties by accounting for interactions between electrons that HF neglects. They are generally more computationally intensive and are chosen according to the balance between desired accuracy and system size. The term covers perturbation theory, configuration interaction, coupled cluster, and multireference approaches, among others.

Prominent families include Møller–Plesset perturbation theory (MP2, MP3, MP4), which adds correlation corrections perturbatively and scales roughly as N^5; Configuration Interaction (CI), including CISD and higher excitations, which is conceptually straightforward but rapidly becomes intractable for large systems and is not size-extensive except in full CI. Coupled-cluster methods, particularly CCSD and CCSD(T), provide a favorable balance of accuracy and size scalability; CCSD(T) is often called the gold standard for single-reference systems. Multireference methods such as CASSCF and CASPT2 or NEVPT2 address systems with near-degeneracy and bond-breaking, but at substantial cost and complexity. More recently, density-matrix renormalization group and quantum Monte Carlo approaches have extended post-HF capabilities to larger active spaces and different properties.

In practice, post-Hartree-Fock techniques are used to obtain accurate thermochemistry, reaction barriers, and spectroscopic constants for small to medium-sized molecules. Their applicability is limited by computational cost and potential convergence issues for strongly correlated systems. Researchers often employ composite approaches, basis-set extrapolation, or hybrid schemes combining post-HF with DFT to balance accuracy and resource demands.