In the last decades, the investigation of electronic excitations has become one of the most studied field in quantum chemistry. In this context, many different computational approaches have been developed to describe the optical absorption of complex systems, characterized by local excitations of selected chromophores embedded in an external environment. The computational strategies to include environmental effects are indeed valuable tools used to bridge the gap between theory and experiment.<br/><br/>In this contribution, we present the extension of our novel multilevel quantum-embedding approach defined in the framework of Density Functional Theory (DFT), named Multilevel DFT (MLDFT), to the calculation of excitation energies of large and complex molecular systems. Our approach is defined in the molecular orbital (MO) basis of the active fragment (the chromophore) only. This feature automatically allows for a saving in the computational cost, because the inactive MOs are not involved in the self consistent field (SCF) procedure. Such a feature is also exploited in the extension of the method to Time-Dependent DFT (TDDFT). Here, we apply the developed method to the electronic excitation of selected test cases. The obtained results are compared with full DFT and experimental data.

Electronic excitations of complex molecular systems using multilevel density functional theory

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