In an article currently in press in J. Am. Chem. Soc., R. B. Gerber and graduate student Yemi Adesokan, in collaboration with the experimental group of Richard Mathies and co-workers at UC Berkeley, were able to compute with high accuracy vibrational frequencies of the major intermediates of the Photoactive Yellow Protein (PYP) photocycle. The calculations use ab initio potentials and employs an algorithm that goes beyond the harmonic approximation. The results are an important step forward for first-principles calculations of vibrational spectroscopy of biological chromophores, including of short lived intermediates. Insight into the evolution of the potential surface along the reaction path is also obtained.