Introduction
Coester and Kummel first developed the ideas that led to coupled-cluster
theory in the late 1950s. The basic idea is that the correlated wave function
of a many-body system \( \mid\Psi\rangle \)
can be formulated as an exponential of correlation
operators \( T \) acting on a reference state \( \mid\Phi\rangle \)
$$
\mid\Psi\rangle = \exp\left(-\hat{T}\right)\mid\Phi\rangle\ .
$$
We will discuss how to define the operators later in this work. This simple
ansatz carries enormous power. It leads to a non-perturbative many-body
theory that includes summation of ladder diagrams , ring
diagrams, and an infinite-order
generalization of many-body perturbation theory.