Electronic structure of ionized non-covalent dimers: Methods development and applications

Several prototypical ionized non-covalent dimers - the uracil, 1,3-dimethylated uracil and benzene dimer cations - are studied by high-level ab initio approaches including the equation-of-motion coupled cluster method for ionization potentials (EOM-IP-CC). The qualitative Dimer Molecular Orbitals as Linear Combinations of Fragment Molecular Orbitals (DMO-LCFMO) framework is used to interpret the results of calculations.;As the simplest model systems, the neutral and ionized non-covalent dimers, such as pi-stacked and H-bonded nucleobase dimers, can shed some light on the complex mechanism of the charge transfer in DNA. The correct treatment of non-covalent interactions is challenging to the ab initio methodology, therefore the special attention is given to the development and benchmarking of the new methods.;First, we introduce and benchmark the cost-saving configuration-interaction variant of the EOM-IP-CCSD method: EOM-IP-CISD. The computational scalling of EOM-IP-CISD in N5, as opposed to the N6 scalling of EOM-IP-CCSD. The EOM-IP-CISD structures for the open-shell systems are of a similar quality as the HF geometries of well-behaved closed-shell molecules, while the excitation energies are of a semiquantitative value. The performance of promising Density Functional Theory developments, i.e. the novel long-range and dispersion-corrected functionals, is also assessed throughout this work.;Next, the potential energy surfaces, electronic structure and properties of uracil dimer and 1,3-dimethylated uracil dimer cations are investigated. The electronic structure of dimers is explained by DMO-LCFMO. Non-covalent interactions lower the vertical ionization energies by up to 0.35 eV, the largest red-shift is observed for the stacked and t-shaped structures. Ionization induces significant changes in bonding patterns, structures and binding energies. In the cations the interaction between the fragments becomes more covalent and the binding energies are 1.5-2.0 times larger than in the neutrals. The relaxation of the cation structures is governed by two different mechanisms: the hole delocalization and the electrostatic stabilization. The electronic spectra of dimer cations exhibit significant changes upon relaxation, which can be exploited to experimentally monitor the ionization-induced dynamics. The position and intensity of the charge-resonance transitions can be used as spectroscopic probes in such experiments. Finally, we investigate the effect of substituents on the electronic structure of non-covalent dimers. For weak perturbations, i.e. the CH3 group, the effect of substituents can be incorporated into the qualitative DMO-LCFMO picture as constant shifts of the dimers and the monomers levels.