Electron affinities of small uracil-water complexes: a comparison of benchmark CCSD(T) calculations with DFT.

We present benchmark CCSD(T) calculations of the adiabatic electron affinities (AEA) and the vertical detachment energies (VDE) of the uracil molecule interacting with one to three water molecules. Calculations with rather large aug-cc-pVTZ basis set were only tractable when the space of virtual orbitals was reduced to about 60% of the full space employing the OVOS (Optimized Virtual Orbital Space) technique. Because of the microhydration, the valence-bound uracil anion is stabilized leading to gradually more positive values of both AEA and VDE with increasing number of participating water molecules. This agrees with experimental findings. Upon hydration by three water molecules, the electron affinity of uracil increased in comparison with AEA of the isolated molecule by about 250 up to 570 meV, depending on the geometry of the complex. CCSD(T) results confirm trends determined by DFT calculations of the microhydrated uracil and its anion, even if electron affinities of the free and hydrated uracil molecule are overestimated by DFT by up to 300 meV.

Toward more efficient CCSD(T) calculations of intermolecular interactions in model hydrogen-bonded and stacked dimers.

Interaction energies of the model H-bonded complexes, the formamide and formamidine dimers, as well as the stacked formaldehyde and ethylene dimers are calculated by the coupled cluster CCSD(T) method. These systems serve as a model for H-bonded and stacking interactions, typical in molecules participating in biological systems. We use the optimized virtual orbital space (OVOS) technique, by which the dimension of the space of virtual orbitals in coupled cluster CCSD(T) calculations can be significantly reduced. We demonstrate that when the space of virtual orbitals is reduced to 50% of the full space, which means reducing computational demands by 1 order of magnitude, the interaction energies for both H-bonded and stacked dimers are affected by no more than 0.1 kcal/mol. This error is much smaller than the error when interaction energies are calculated using limited basis sets.