The geometries of the DNA nucleoside pairs between 2'-deoxyriboadenosine (dA) and 2'-deoxyribothymidine (dT) and its anion (dAdT(-)) were fully optimized using carefully calibrated density functional methods. The addition of an electron to dAdT results in remarkable changes to the two hydrogen bonding distances, the (HO)-O-... distance decreasing by 0.303 angstrom and the (NH)-H-... distance increasing by 0.229 angstrom. The electron affinity of the dAdT pair was studied to reveal the correct trends of adiabatic electron affinity (EA(ad)) under the influence of the additional components to the individual bases. The consequence of negative charge in terms of structural variations, energetic changes, and charge distribution were explored. The EA(ad) of dAdT is predicted to be positive (0.60 eV), and it exhibits a substantial increase compared with those of the corresponding bases A and T and the nucleic acid base pair AT. The effects of pairing and the addition of the sugar moiety on the EAad are well described as the summation of the individual influences. The influence of the pairing on the EA is comparable to that of the addition of 2-deoxyribose. The excess charge is mainly located on the thyminyl moiety in the anionic dAdT pair. The positive vertical electron affinity (VEA = 0.20 eV) for dAdT suggests that it is able to form a stable anion through electron attachment. A large vertical detachment energy (VDE = 1.14 eV) has been determined for the anionic dAdT nucleoside pair. Therefore, one may expect that the stable anionic dAdT nucleoside pair should be able to undergo the subsequent glycosidic bond cleavage process.