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We have taken (dA), (dT), and (dA)·(dT) as model systems to study concerted effects of base pairing and stacking on excited-state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial → charge-transfer nature in the oligonucleotides. The → charge-transfer transitions produce dark states, which are hidden in the energy region of the steady-state absorption spectra. This is different from the previous assignment that the → charge-transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The → charge-transfer states were predicted to have relatively high energies in the oligonucleotides. The present calculations predict that the → charge-transfer states are not involved in the spectra and excited-state dynamics of the oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the nπ* and ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]