The potential energy surface along the hydrogen-bonded proton transfer between the Watson-Crick (WC) adenine-thymine (A-T) base pair of deoxyribonucleic acid (DNA) and its tautomeric structures is calculated with 6-31G(d,p) basis set in Hartree-Fock (HF), density functional theory with Becke's three-parameter hybrid Lee-Yang-Parr exchange-correlation functional (B3LYP), second order Møller-Plesset perturbation (MP2), and coupled cluster singles and doubles (CCSD) levels. The tautomeric structure, where both two hydrogenbonded protons in the A-T base pair have transferred each other, is found at all level of calculations. Though the optimized structure in which only one hydrogen-bonded proton in adenine has migrated to thymine is found at HF level, we could not obtain such optimized structure at both MP2 and B3LYP levels. Including electron correlations, the energy differences between the canonical A-T and the two hydrogenbonded protons transferred tautomeric structure become smaller. Aside from this, potential energy surface from the WC A-T to the Hoogsteen type A-T gives almost the same among each level of calculation.