Excitonically renormalised coupled-cluster theory

ABSTRACT

A variant of coupled-cluster theory is described here, wherein the degrees of freedom are fluctuations of fragments between internally correlated states. The effects of intra-fragment correlation on the inter-fragment interaction are pre-computed and permanently folded into an effective Hamiltonian, thus avoiding redundant evaluations of local relaxations associated with coupled fluctuations. A companion article shows that a low-scaling step may be used to cast the electronic Hamiltonians of real systems into the form required. Two proof-of-principle demonstrations are presented here for non-covalent interactions. One uses harmonic oscillators, for which accuracy and algorithm structure can be carefully controlled in comparisons. The other uses small electronic systems (Be atoms) to demonstrate compelling accuracy and efficiency, also when inter-fragment electron exchange and charge transfer must be handled. Since the cost of the global calculation does not depend directly on the correlation models used for the fragments, this should provide a way to incorporate difficult electronic structure problems into large systems. This framework opens a promising path for building tunable, systematically improvable methods to capture properties of systems interacting with a large number of other systems. The extension to excited states is also straightforward.

GRAPHICAL ABSTRACT

KEYWORDS:

• Fragment methods
• electron correlation
• excitons
• renormalisation
• effective hamiltonian
• range separation
• coupled cluster

Acknowledgements

The authors gratefully acknowledge start-up support from the Hornage Fund at the University of the Pacific, as well as equipment and travel support provided by the Dean of the College of the Pacific. The following colleagues are recognised for useful insights during the development of this work: Arindam Chakraborty, Gregory J. O. Beran, Oriol Vendrell, Andreas Dreuw, Joshua Schrier.

Disclosure statement

No potential conflict of interest was reported by the author(s).