A quick tour of Coupled Cluster theory
Coupled cluster theory has several interesting computational features and is the method of choice in quantum chemistry. The method was originally proposed by Coester and Kummel, two nuclear physicists (way back in the fifties). It came back in full strength in nuclear physics during the last decade.
There are several interesting features:
- With a truncation like CCSD or CCSDT, we can include to infinite order correlations like \( 2p-2h \).
- We can include a large basis of single-particle states, not possible in standard FCI calculations
However, Coupled Cluster theory is
- non-variational
- if we want to find properties of excited states, additional calculations via for example equation of motion methods are needed
- if correlations are strong, a single-reference ansatz may not be the best starting point
- we cannot quantify properly the error we make when truncations are made in the cluster operator