The molecular mechanism for removing the excess energy in the bases is responsible for the high photostability of DNA and is thus the subject of intensive theoretical/computational investigation.To understand why the excited state decay of the stacked bases is significantly longer than that of the monomer, we carried out electronic structure calculations on an adenine monomer and an aqueous (dA)5 oligonucleotide employing the CASPT2//CASSCF and CASPT2//CASSCF/Amber levels of theory.