(a1, b1, c1) and (a2, b2, c2), one frame corresponding to the entire water molecule: (x, y, z), and the laboratory frame: (X, Y, Z). In the two O-H frames (Figure ), the corresponding c1-axis and c2axis point along the O-H bonds. In the molecular frame (Figure ), the z-axis points along the bisector of the H-O-H angle and the x-axis is in the H-O-H plane. In the laboratory frame, the Z axis is perpendicular to the interface defining the uniaxial system and the X and Y axes point along the surface.