Molecular dynamics (MD) simulations were carried out to obtain the conformational changes of the chromophore in the gold fluorescent protein (PDB ID: 1OXF). To obtain two-photon absorption (TPA) cross-
sections, time dependent density functional theory (TD-DFT) computations were performed for chromophore geometries sampled along the trajectory. The TD-DFT computations used the CAM-B3LYP functional and 6-31+G(d) basis set. Results showed that two dihedral angles change remarkably over the simulation time. TPA cross-sections were found to average 13.82 GM for the excitation to S1 computed from the equilibrium geometries; however, extending the structures with a water molecule and GLU residue, which make H bonds with the chromophore molecule, increased excitation energies and TPA cross- sections significantly. Besides the effects of the surrounding residues and the dihedrals on the spectroscopic properties, some bond lengths affected the excitation energies and the TPA cross-sections significantly (up to ±25−30%), while the effects of the bond angles were smaller (±5%). Overall the present results provide insight into the effects of the conformational flexibility on TPA (with gold fluorescent protein as a specific example) and suggest that further experimental measurements of TPA for the gold fluorescent protein should be undertaken.