We investigate the S 1 state potential energy surface of 2-pyridone dimer (2PY)2 using time-dependent density functional and coupled cluster theory. Although the ground and S 2 excited states of (2PY)2 have C 2h symmetry, the S 1 state shows symmetry breaking and localization of the excitation on one of the two monomers upon relaxation of the geometry. This localization is rationalized using a simple diabatic curve crossing model. As a consequence of the symmetry breaking, S 1 to S 0 transitions become optically allowed. We hypothesize that the band at 30 776 cm−1 observed in the excitation spectrum of (2PY)2 might be attributed to the S 1 state rather than the S 2 state; the S 2 state origin is predicted 3000−4000 cm−1 above the S 1 state by hybrid density functional and coupled cluster methods. Asymmetric transfer of one hydrogen atom leads to a second S 1 state minimum that can rapidly decay to the ground state. This suggests that photoinduced tautomerization of (2PY)2 occurs in a stepwise fashion, with only one hydrogen transfer taking place on the S 1 surface.