A quick tour of Coupled Cluster theory
In our FCI discussion the correlation energy is defined as, with a two-body Hamiltonian,
ΔE=∑ai⟨i|ˆf|a⟩Cai+∑abij⟨ij|ˆv|ab⟩Cabij.
In Coupled cluster theory it becomes (irrespective of level of truncation of T)
ΔE=∑ai⟨i|ˆf|a⟩tai+∑abij⟨ij|ˆv|ab⟩tabij.