The proton transfer from the green fluorescent protein chromophore to a nearby water molecule is studied by means of CASSCF, CASPT2 and TDDFT calculations. A (1)pisigma* electronic state is found to intersect with the photoactive (1)pipi* electronic state along the proton transfer coordinate. This state crossing constitutes a possible non-radiative deactivation pathway of the photoexcited neutral form of the chromophore. A discussion on the performance of the different levels of theory employed is also given, focusing in the ability to correctly describe the (1)pisigma* electronic state. (C) 2004 Elsevier B.V. All rights reserved.