The Wick theorem for coupled fermion clusters

Alexei M. Tsvelik

B. Westwański

Yung‐Li Wang, S. Shtrikman, Herbert B. Callen

It is shown that Wick's theorem and the linked diagram expansion theorem for S = ½ operators (applied elsewhere by the authors to discuss antiferromagnetic spin waves) can be derived by a direct application of conventional theorems to the coupled-fermion representation of spin operators.

V. A. Malyshev

Frank E. Harris, Bogumił Jeziorski, Hendrik J. Monkhorst

A proof by induction is given of the so-called contraction theorem for the evaluation of (anti)commutators of strings of Fermion creation and annihilation operators. This theorem bears some formal similarity to Wick's theorem but is essentially simpler and its applications do not lead to any disconnected diagrams. Examples of applications to configuration-interaction and coupled-cluster methods are presented.

Dieter Ihle