Configuration interaction ($\Psi ={\sum }_I{c}_I{\Phi }_I$)

A variational expansion $\Psi ={\sum }_I{c}_I{\Phi }_I$ over Slater determinants or spin-adapted configuration functions in a fixed orbital basis. Truncating the configurations restricts the correlation described.

configuration interaction retaining a reference determinant and its single and double excitations. Conventional matrix-vector iterations have leading $O(o^2{v}^4)$ work for occupied and virtual dimensions $o,v$, hence $O(N^6)$ when both grow with system size. Truncated CI is not generally size extensive.