Nonlinear Spectroscopy and Magnetic Circulardichroism with the Approximate Coupled-Cluster Singles-and-Doubles Method RI-CC2

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In a second funding period the implementation of perturbation-induced transition moments for RI-CC2, developed in the first period, should be extended in two directions:firstly towards transition moments induced by magnetic fields, to describe magnetic circular dichroism (MCD), and secondly towards transitions between two excited states including spin-orbit couplingfor transitions between singlet and triplet states.MCD spectroscopy is experimentally often applied to obtain additional information which complements the results form UV/Vis absorption spectra since bands which overlap in the absorption spectrum can have opposite signs in the MCD spectrum. For closed-shell organic molecules the MCD spectrum is dominated by the so-called B term which describes a change in the oscillator strengths induced by the magnetic field. Despite the experimental importance of MCD spectroscopy theoretical computations of the MCD spectra of organic molecules are currently limited to density functional theory, which has knowndefiencies for charge-transfer and Rydberg excitations, or at correlated wavefunction levels to rather small molecules.An efficient implementation of MCD spectra for the approximate coupled-cluster singles-and-doubles model CC2 with the resolution-of-the-identity approximation (RI-CC2) would greatly extend the possibilities for theoretical studies of MCD at a correlated wavefunction level.