The ground and excited state potentials and force field of the copper ion site of plastocyanin in the oxidized state is studied by ab initio electronic structure calculations, with the aim to explore the mechanism of its photodynamics. It is shown that the potential energy surface of the ligand-to-metal charge-transfer (LMCT) state, which corresponds to the intense similar to600 nm absorption, is repulsive along the Cu-S(Cys) distance, where S(Cys) denotes the sulfur atom of the cystein ligand, and crosses with the d --> d ligand field excited states at the Cu-S(Cys) length of similar to2.7 Angstrom or longer. The crossing distance varies depending on the Cu-N(His) distance of the two histidine ligands, indicating that the Cu-His ligand vibrational motions, in addition to the Cu-S(Cys) stretch, constitute a major component of the reaction coordinate for the nonradiative transitions. This feature is explained in terms of the different charge distributions and the resulting electrostatic interactions in these states. The influence from the methionine ligand is seen to be minor, although the possible dynamical coupling via the dispersion interactions is not ruled out. A shorter Cu-S(Met) equilibrium distance (2.4 Angstrom) than in the crystal structure (2.9 Angstrom) is found, suggesting that the structural constraint from the surrounding protein may be playing some role.