| Summary: | Because electrons interact pairwise and because all electrons are indistinguishable from one another, the energy of an N-electron system can be described, without approximation, as a functional of the two-electron reduced density matrix (2-RDM). While the N-electron wavefunction and density matrix grow exponentially with system size, the 2-RDM only grows polynomially. Direct determination of the 2-RDM without knowledge of the N-electron wavefunction can therefore provide a computationally advantageous route for the calculation of molecular properties. This work focuses on the characterization of multireferencemolecular systems, which are systems that cannot be described by a single Slater determinant. Such systems often prove challenging for simple characterization by electronic structure methods. Two different 2-RDM methods are applied to a variety of multireference systems: the parametric 2-RDM (p2-RDM) method and the solution to the anti-Hermitian contracted Schrödinger equation (ACSE). The p2-RDM method is applied to a number of diradical systems. While relying on only a single Slater determinant, the parametric 2-RDM method is able to give results typically only found by multireference methods. More general multireferencesystems are treated using the ACSE. A new algorithm for solving the ACSE is presented which provides a 5-20 times speedup in convergence time. This has enabled the study of considerably larger systems. We investigate how the ACSE and other multireference methods are affected by the quality of the underlying reference calculation by studying the ground and excited states of a number of π-conjugated systems. Lastly, the excited state electronic structure and exciton dynamics in fluorone-based chromophore dimer systems are explored. The relationship between electronic structure modification via chemical substituents and excitondynamics is studied, highlighting important principles for the future design of efficient synthetic light-harvesting systems.
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