postHartreeFockberegninger

Post-Hartree-Fock calculations are a class of methods in quantum chemistry used to improve upon the results obtained from the Hartree-Fock (HF) method. The Hartree-Fock method provides a fundamental approximation by treating electron-electron interactions in an averaged manner, neglecting instantaneous correlations between electrons. This simplification leads to a ground-state energy that is an upper bound to the true energy but is often not sufficiently accurate for quantitative predictions.

Post-Hartree-Fock methods aim to account for electron correlation, which is the deviation from the independent-electron approximation. These methods systematically add corrections to the HF energy to recover the missing correlation energy. Common examples include configuration interaction (CI), Møller-Plesset perturbation theory (MPn), and coupled cluster (CC) theory.

Configuration interaction involves expressing the true wavefunction as a linear combination of excited determinants built from the HF reference determinant. Full CI, which includes all possible excitations, is exact but computationally intractable for all but the smallest systems. Truncated CI methods, such as CISD (singles and doubles) or CISDT, include only a limited number of excited configurations.

Møller-Plesset perturbation theory treats the difference between the exact Hamiltonian and the Fock operator as a perturbation. The correlation energy is then calculated as a series expansion in terms of the perturbation. MP2 is the simplest and most widely used level of MP theory.

Coupled cluster theory, particularly CCSD (singles and doubles) and CCSD(T) (which adds a perturbative correction for triples), is known for its high accuracy and favorable scaling with system size compared to truncated CI methods, making it a popular choice for accurate molecular property calculations. These methods, despite their computational cost, are essential for obtaining reliable results in many areas of computational chemistry.