Polarizable Density Embedding Coupled Cluster Method.

We present the theory and implementation of the polarizable density embedding (PDE) model in combination with coupled cluster (CC) theory (PDE-CC). This model has been implemented in the Dalton quantum chemistry program by adapting the CC code to the polarizable embedding library ( PElib). In the PDE-CC method, the smaller, but chemically important core region is described with a high-level CC method. The environment surrounding the core region can be separated into two levels of description: an inner and an outer region. The effect of the inner region on the core region is described by an embedding potential consisting of a set of fragment densities obtained from calculations on isolated fragments with a quantum-chemistry method such as Hartree-Fock (HF) or Kohn-Sham density functional theory (KS-DFT) and dressed with a set of atom-centered anisotropic dipole-dipole polarizabilities. The outer region consists of distributed atom-centered multipoles and polarizabilities, i.e., in the same way as in the polarizable embedding (PE) model. The PDE-CC method contains embedding potential operators that account for the electrostatic and polarization interactions between the core region and the environment, as well as for nonelectrostatic (also known as Pauli and exchange) repulsion. All environmental effects are included through one-electron operators and account very efficiently for the response of the environment due to the change in the electron density of the core region, e.g., upon an electronic transition.