Several theoretical methods are employed to characterize the intramolecular proton transfer in the ground state and in the lowest-lying 3nπ∗ and 3ππ∗ excited states of 1-amino-3-propenal. The geometrical parameters, the relative energy of the two tautomeric forms, the energy barrier for the proton transfer, and the energy difference between the ground and the excited states predicted by the different methods are compared. It was found that: (1) the CASPT2 results are in good agreement with those obtained using the CCSD(T) method; (2) the CIS method and the CASSCF method with a medium-sized active space yield poor geometries and overestimate the adiabatic energy excitations and the energy barriers for the proton transfer; and (3) the B3LYP method provides good adiabatic excitation energies, although B3LYP energy barriers are Correspondence to: M. Solà; e-mail: miquel@iqc.udg.es Contract/grant sponsor: Spanish DGICYT; contract/grant numbers: PB95-0762 and PB98-0457-C02-02 Contract/grant sponsor: European Union; contract/grant number: CI1*-CT93-0339 Contract/grant sponsor: Generalitat de Catalunya; contract/grant number: CIRIT FI/96-05011.1 This article includes Supplementary Material available from the authors upon request or via the Internet at ftp.wiley. com/public/journals/jcc/suppmat/21/257 or http://journals.