We use ab initio results to reparameterize a multi-reference semiempirical method to reproduce the ground and excited state potential energy surfaces (PESs) for the chromophore of Green Fluorescent Protein (GFP). The validity of the new parameter set is tested, and the new method is combined with a quantum mechanical/molecular mechanical (QM/MM) treatment so that it can be applied in the solution phase. Solvent effects on the energetics of the relevant conical intersections are explored. We then combine this representation of the ground and excited state PESs with the full multiple spawning (FMS) nonadiabatic wavepacket dynamics method to simulate the photodynamics of the neutral GFP chromophore in both gas and solution phases. In these calculations, the PESs and their nonadiabatic couplings are evaluated simultaneously with the nuclear dynamics, ie. "on-the-fly". The effect of solvation is seen to be quite dramatic, resulting in an order of magnitude decrease in the excited state lifetime. We observe a correlated torsion about a double bond and its adjacent single bond in both gas and solution phases. This is discussed in the context of previous proposals about minimal volume isomerization mechanisms in protein environments.

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.

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