Frozen density embedding with pCCD electron densities

Abstract: The pair-coupled-cluster doubles (pCCD) method has emerged as a viable approach for quantum-chemical studies of strongly correlated systems. Despite its lower formal scaling (O(N$^4$)) compared to other versions of coupled cluster (CC) theory, applications to large chemical structures are still expensive. Fragmentation and embedding strategies offer a viable approach in such cases. In this work, we present a simple and efficient density-embedding scheme based on pCCD electron densities. The main computational benefit arises from the fact that pCCD response $Λ$-equations are much cheaper to compute than those of standard CC methods, providing easy access to one-electron properties. The pCCD densities of the individual subsystems are used to generate static embedding potentials that capture the environment's effect on the embedded system. The individual fragment energies are then iteratively converged in a self-consistent fashion. We demonstrate the reliable performance of this scheme with the estimation of dipole moments of the weakly bound CO2$\cdots$Rg (Rg = He, Ne, Ar, and Kr) complexes and with the modeling of vertical excitations of some microsolvated molecules.