C. Ko, B. Levine, A. Toniolo, L. Manohar, S. Olsen, H.-J. Werner, and Todd J. Martinez

The photoisomerization mechanism of the neutral form of the photoactive yellow protein (PYP) chromophore is investigated using ab initio quantum chemistry and first-principles nonadiabatic molecular dynamics (ab initio multiple spawning or AIMS). We identify the nature of the two lowest-lying excited states, characterize the short-time behavior of molecules excited directly to S2, and explain the origin of the experimentally-observed wavelength-dependent photoisomerization quantum yield.