Modelling one- and two-photon spectroscopy in solution with the coupled-cluster model CC2 combined with continuum and atomistic embedding models

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The aim of the project is the development of an efficient and general implementation of the Conductor-like screening model (COSMO) and the atomistic polarizable embedding model (PE) for the coupled-cluster method CC2 and the related ADC(2), CIS(Dinf) and CIS(D) methods including their spin-component scaled SCS and SOS variants. For the implementation we will exploit that both approaches are special cases of a self-consistent reaction field model and can be casted into a common work flow and program structure. In both approaches the interaction between the electrons and the environment is described through an embedding or reaction field potential which is a one-electron operator and a functional of the electron density in the quantum system. In both cases the changes of the embedding potential upon changes in the quantum system as response to an externally applied electromagnetic field should be included. For continuum solvation models like COSMO this was shown in Refs. [1,2]. For the PE model the response contributions to the embedding potential were already included by construction in its original formulation [3].The one-electron form of the embedding potential and its dependence on the density and its response to perturbations determines the algorithms and the workflow of the embedded electronic structure calculation. The implementation should comprise in addition to vertical excitation energies and intensities for one- and two-photon UV/Vis transitions also rotatory strengths for one- and two-photon circular dichroism, phosphorescence lifetimes, and first-, second- and third-order properties and gradients in the electronic ground and in excited states. It should be integrated into the MPI and OpenMP parallel versions for use on distributed and shared memory computer systems. The limitations of already existing program parts to closed-shell Hartree-Fock references should be removed.To validate the two embedding approaches, a test set with experimental and theoretical reference data should be compiled and the performance of the COSMO and PE embedding models in the interplay with the correlated wavefunction methods CC2, ADC(2), CIS(Dinf), and CIS(D) benchmarked for the implemented properties.