Investigation and modelling of dispersion interactions in electronically excited states and the effects of dispersion interactions on electronic excitations

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The aim of the project is to achieve a better understanding of the connection between the electronic excitations in a chromophore and its dispersion interaction with an environment. Important research questions that will be addressed are: In which chemical situations and for which kind of electronically excited states is it important to account for the change of the dispersion interaction upon electronic excitation in its quantum chemical description? Are there situations and/or chromophores where the dispersion interaction with the environment changes significantly the excitation energy or the electronic structure of the excited state or the order of the excited states?An additional object of the project is the development of the dispersion contribution to the self-consistent reaction field model PE (polarizable embedding) which accounts for the dispersion interaction during the calculation of the wavefunction parameters for the ground and electronically excited states. A posteriori corrections will thereby be avoided because they lead in general to qualitatively wrong potential energy surfaces in the vicinity of avoided crossings. This is done because we aim at a QM/MM method, which includes the dispersion interaction upon electronic excitation processes and is suitable for the calculation of excited state structures and relaxation processes. This will be implemented at the example of the PERI-ADC(2) and PERI-CC2 methods.For the computation of dispersion coefficients for larger molecules with a correlated wavefunction method we will in addition extend an existing RI-CC2 code for frequency-dependent polarizabilities in ground and excited states for imaginary frequency and for the calculation of Cauchy moments.