Excitonic Coupled-cluster Theory

TL;DR

This paper introduces a fragment-based coupled-cluster method that efficiently captures inter-fragment interactions by pre-computing intra-fragment correlations, enabling scalable and accurate modeling of large molecular systems.

Contribution

It develops a novel excitonic coupled-cluster approach that incorporates intra-fragment correlations into an effective Hamiltonian, reducing computational cost for large systems.

Findings

Accurate modeling of non-covalent interactions demonstrated.

Efficient handling of electron exchange and charge transfer.

Potential for scalable, systematically improvable electronic structure methods.

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.