TS and DOS for the AT nucleobase pair. (a) TS, (b) DOS, (c) molecular orbital (MO) analysis for HOMO and LUMO, where the charge separation is clearly shown between HOMO and LUMO, (d) atomic orbital (AO) analysis. The 2px, 2py, and 2pz orbitals are shown for the N-atom and O-atom. It clearly shows the overlap of the 2py orbitals.