The bending of certain lines is due to configuration interactions of the excited states.

This notation means that the corresponding Slater determinants have a clear higher weight in the configuration interaction expansion.

Ideally we should include configuration interaction (CI), but this is rarely possible except for small molecules.

In comparison, a configuration interaction calculation may require around 50,000 parameters to reach that accuracy, although it depends greatly on the particular case being considered.

Douglas recognized the importance of configuration interaction that he referred to as "superposition of configurations".

Full configuration interaction with Slater determinants (benchmark studies)

Therefore, coupled cluster terminated at usually recovers more correlation energy than configuration interaction with maximum n excitations.

Necessarily this is an approximation, and highly accurate descriptions of the molecular electronic wave function do not have orbitals (see configuration interaction).

Moreover, it sometimes happens that the configuration interaction expansion converges very slowly and that one cannot speak about simple one-determinant wave function at all.

His name is associated with both the Davidson correction and the "Davidson diagonalization" method which he applied to configuration interaction methods.

Configuration interaction (CI)

The error in CISD calculations can be corrected with e.g. quadratic configuration interaction.

The Davidson correction is a simple correction which allows one to estimate the value of the full-CI energy from a limited configuration interaction expansion result.

Quadratic configuration interaction (QCI)

The choice of the exponential ansatz is opportune because (unlike other ansätze, for example, configuration interaction) it guarantees the size extensivity of the solution.

Quantum chemistry methods, Møller-Plesset perturbation theory, coupled cluster method or full configuration interaction can give extremely accurate results, but require large computational cost.

Multireference configuration interaction (MRCI)

Configuration interaction calculations are based on Graphical Unitary Group Approach (GUGA).

In more accurate theories (such as configuration interaction and MCSCF), a linear combination of Slater determinants is needed.

The methods used were mostly post-Hartree-Fock, particularly configuration interaction (CI) and coupled cluster (CC).

It is capable of calculating various molecular properties using the Hartree-Fock, MP2, density functional theory, configuration interaction and coupled cluster electronic structure theories.

Multireference single and double configuration interaction (MRCISD)

The emphasis in the program is on highly accurate computations, with extensive treatment of the electron correlation problem through the multireference configuration interaction, coupled cluster and associated methods.

Instead, there are serious reports like Multireference configuration interaction (MRCI), which accurately and thoroughly explain the observed phenomena by means of the fundamental interactions.

Of the common quantum mechanical methods Hartree-Fock, coupled cluster, many-body perturbation theory (to any order), and full configuration interaction (CI) are size consistent.