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Molecular Gradients of Polarizable Embedded RI-CC2

Alireza Marefat Khah (Department of Theoretical Chemistry, Ruhr University of Bochum)
Christof Haettig (Department of Theoretical Chemistry, Ruhr University of Bochum)

During the last decades quantum chemistry becomes advanced enough to simulate
various molecular properties (energy, spectroscopic properties, etc.) for
medium-sized molecules with acceptable accuracy in vacuum. While these methods
are being developed all over the globe, simulation of experimental observables
requires enhancing computational methods to calculate different molecular
properties beyond the vacuum. An atomistic model of molecular environments is
the Polarizable-embedding approach which is combined with the HF, the TDDFT [1]
and the Coupled Cluster theory [2]. In this approach, molecular environment are
partitioned to polarizable point charges embedding the quantum region and the
interaction between the quantum system and environmental point charges are
optimized in a self-consistent way. Therefore the molecular structure of
environment (e.g. solvent, protein, surface, …) is kept unchanged and the
specific electrostatic interactions could be described properly for a given
configuration of the system. Within an undergoing project, the existing
PERI-CC2 implementation for excitation energies and one- and two-photon
transition moments is extended for molecular gradients for ground and excited
states. To arrive at a neat derivation of the working equations we base the
formulation of the theory on Lagrangians for the ground and excited state
energies and one- and two-particle densities. Using a recursive approach, the
multipole moments interactions of the embedding sites are included up to an
arbitrary order while the dispersion interaction between the QM and MM region
are described with a Lennard-Jones potential as implemented for PE-DFT [3].
Finally, the differences between the expressions for PERI-CC2, vacuum RI-CC2
and COSMO-CC2 will be briefly discussed.

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