A coupled cluster-based wave function-in-wave function quantum embedding theory

We have recently developed a novel quantum embedding framework (J. Chem. Phys. 161, 164107 (2024)), which employs: a) coupled cluster (CC) theory with a specific truncation in the cluster operator as a high-level solver and b) a perturbative method with an efficient treatment of orbital relaxation as a low-level method. The framework exhibits qualitative similarities to the all-electron CC method for symmetry-breaking problems arising from bond stretching in molecules when unrestricted references are used. Moreover, it demonstrates rapid convergence (to all-electron CC) with respect to increased fragment size and quite notably avoids basis set artifacts, which is often a problem for other embedding approaches.

In this presentation, we will first demonstrate that the method can be implemented using an efficient O(N⁴) algorithm. This is achieved by utilizing an approximate static renormalized interaction for the fragment problem, utilizing the quantities obtained from the low-level method. Moreover, the method exhibits linear scaling with respect to the number of fragments. We have leveraged this property to enhance the current method for multiple fragments. This allows us to investigate various clusters within a molecule or a molecular aggregate.

We will then discuss our effort to increase the accuracy of the current approach beyond the singles doubles approximation of the CC theory. This will have an impact on the complicated open-shell problems where “gold standard” CCSD(T) method is inadequate, such as radical chemistry, and/or precise calculation of barrier heights and non-covalent interaction energies. In this direction, I will present a few studies involving transition metals.