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Abstract

During the last three decades, the field of theoretical and computational chemistry has evolved rapidly, promoted by the increasing availability of computational power inherent in modern CPUs and cluster structures. This development has expressed itself in a particular manner in the formulation of modern state-of-the-art electronic structure methods in the frameworks of, for instance, the Algebraic Diagrammatic Construction scheme or Coupled Cluster. Application of these methods to the calculation of molecular excitation energies and properties that describe the most fundamental processes of light-matter interaction, has been established as a profound and reliable tool for experimentalists within chemistry and molecular sciences. This work, which is spilt into three main parts, presents the implementation as well as benchmarking of novel electronic structure methods for the Algebraic Diagrammatic Construction scheme (ADC) as well as Unitary Coupled Cluster (UCC). In the first part, an implementation and benchmark study for the calculation doubly-ionized as well as double electronically-attached states for ADC, termed DIP-ADC and DEA-ADC, respectively, up to third order is presented. The implementation was executed in the Q-Chem program package, benchmark studies included the comparison of states to Full CI data for DIP as well as EOMDIP-CCSD and EOMDEA-CCSD. For both schemes, the third-order methods DIP-ADC(3) as well as DEA-ADC(3) proved to produce results which are in a remarkable good agreement to the corresponding EOM-CCSD method. As for the second part, a benchmark study on core excitation energies in the framework of Unitary Coupled Cluster was presented. To this end, the Core-valence separation approximation was applied to the second-and third-order UCC schemes for electron excitations. It was shown that CVS-UCC is very suitable in the computation of X-ray spectra, similar to CVS-ADC, which has been studied before by Wenzel et al. and that it provides reliable data for the description and simulation of energetically high-lying core excitations. The last part of this work features an implementation of an automated code generator for Unitary Coupled Cluster as it was realized by Leitner et al. for the computation of electronically excited states on a ADC(4) level. Working equations for a UCC3-x scheme (in similarity to UCC2-x with an extended description of the doubles/doubles block of the secular matrix) as well as a full UCC4 for the calculation of electronically-excited states and properties are presented, together with an improved ground-state description, termed UCC4+5[s,t] that includes fifth-order terms. At the end of this work, all results for the three presented topics are summarized in detail, accompanied by a short outlook of what could include future work and development.