Chapter 42 - How and why coupled-cluster theory became the pre-eminent method in an ab into quantum chemistry

Abstract

The correlation problem has been a focal point of quantum chemistry for more than 40 years. Coupled-cluster methods typically offer its best numerical solution for molecules and atoms in terms of rigor and systematic convergence to the right answer. Coupled-cluster (CC) theory is not variational, but, more importantly, it is size-extensive making energy differences meaningful and applications to extended systems like polymers and solids possible. CC introduces higher excitations much more effectively and at less expense than analogous configuration interaction methods. As a nonvariational method, analytical gradients in CC theory require new developments compared to other methods. These new developments emphasize the role of the CC functional that underlies the CC theory for energies, forces, density matrices, and properties of ground and equation-of-motion (EOM-CC) excited states. The latter also permits facile applications for single, doubly, etc. ionized and electron-attached states. The evolution of the critical ideas that lead to coupled-cluster theory and its extensive applications in chemistry are enumerated, in an objective, but personal, first-hand account.