TDDFT, RI-CC2, and CIS calculations have been performed for the nondissociative excited-state proton transfer (ESPT) in the S-1 state of 7-hydroxy-4-methylcoumarin (7H4MC) along a H-bonded water wire of three water molecules bridging the proton donor (OH) and the proton acceptor (CO) groups (7H4MC center dot(H2O)(3)). The observed structural reorganization in the water-wire cluster is interpreted as a proton-transfer (PT) reaction along the H2O solvent wire. The shift of electron density within the organic chromophore 7H4MC due to the optical excitation appears to be the driving force for ESPT. All the methods used show that the reaction path occurs in the (1)pi pi* state, and no crossing with a Rydberg-type (1)pi sigma* state is found. TDDFT and RI-CC2 calculations predict an exoergic reaction of the excited-state enol-to-keto transformation. The S-1 potential energy curve reveals well-defined C-s minima of enol- and keto-clusters, separated by a single barrier with a height of 17-20 kcal/mol. After surmounting this barrier, spontaneous PT along the water wire is observed, leading without any further barrier to the keto structure. The TDDFT and RI-CC2 methods appear to be reliable approaches to describe the energy surfaces of ESPT. The CIS method predicts an endoergic ESPT reaction and an energy barrier, which is too high.