Fig. 5. Two-dimensional potential energy surface (2D-PES) of the shared proton as a function of the distances from the O atoms of the Asp298 carboxy groups (x = d(OD1, H), y = d(OD2, H)). The contour lines are drawn at intervals of 5 kcal mol⁻¹ relative to the energy at (x, y) = (1.0, 2.3), corresponding to the 0⁻ state. The 2D-PES was discretized with a mesh of dx = 0.15 and dy = 0.15 Å intervals.