We used a combined
quantum mechanical/mechanical molecular (QM/MM)
method to investigate ionization and electron attachment for nucleobases
in water. We first explored the snapshot number and QM-region size
to get converged results. The computations of 20 snapshots can get
converged physical quantities and converged QM regions contain about
100 atoms. We computed the vertical ionization energies (VIEs), adiabatic
ionization energies, vertical electron affinities, adiabatic electron
affinities, and vertical detachment energies of five bases in the
explicit aqueous solvent. The computational VIEs of cytosine and thymine
agree well with the experimental values in aqueous solutions. The
results of the five physical quantities indicate that the QM-region
polarization by bulk water evidently affects the ionization and electron
attachment. Holes or excess electrons do not entirely localize on
nucleobases in the ionization and electron attachment. The QM-region
polarization and the movements of the QM atoms induce a drastic decrease
(∼2.1 eV for cation and ∼2.9 eV for anion) of the cationic
and anionic energies in structural relaxations. The potential energy
surfaces of the cation and anion would become very steep near the
equilibrium structure.
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