The potential energy (PE) functions of the electronic ground and lowest pi pi* excited singlet states of the hydrogen-bonded cyclic complex of 2-pyrimidinone with water were theoretically investigated along the proton transfer (PT) reaction coordinate, The full geometry optimization was performed along the PT reaction path. In the geometry optimization the Hartree-Fock method and the configuration interaction method with single excitations (CIS) were used. The energy calculations at the optimized geometries were performed with the complete-active-space self-consistent-field (CASSCF) method and with the second-order perturbation theory, employing the CASSCF wave function as the reference. For the ground state, calculations were also performed with the Moller-Plesset second-order perturbation theory (MP2). We found that the hydroxy form of the 2-pyrimidinone:water complex is stable in the ground electronic state while the hydroxy-to-oxo transformation reaction of the complex is by about 0.67 eV exothermic on the lowest (1) pi pi* excited state PE surface. However, there is a barrier of about 0.19 eV along the PT reaction path on this surface. The top of the barrier is below the energy of the vertical excitation S-0 --> (1) pi pi*; thus, the photoexcited system has sufficient excess energy for the PT reaction to occur spontaneously.