Extended Data Fig. 4: Determining the tilting stiffness of the standing molecule.

a, Approach along the y direction as defined in Fig. 2a, b. Parameters are d = 3.25 Å, l = 12.9 Å and 2a = 4.55 Å; d + l ≠ z, because the tip height z is measured from the centre of the uppermost surface layer, whereas l is measured from the centre of the two silver adatoms in the PTCDA + 2Ag complex. b, Approach along the x direction as defined in Fig. 2a, b. The sketch on the left shows a side view, whereas on the right a perspective view onto the top of the molecule is drawn. c, Left, best fit (green line) of the experimental F _x (on x axis, red circles), obtained with the model in equation (8) and κ _θ  = 630 zN m rad⁻¹ (κ = 0.38 N m⁻¹). Black data points display F _y (on y axis), fitted with equation (4) (blue line). d, As in c, but for a simulated curve (green) with κ _θ  = 20.0 aN m rad⁻¹ (κ = 12.02 N m⁻¹), which is too stiff to reproduce the experimental F _x (red). e, As in c, but for a simulated curve (green) with κ _θ  = 310 zN m rad⁻¹ (κ = 0.19 N m⁻¹), which is too soft. For a detailed discussion of this figure, see Methods. Tilt angles θ_eq and linear elongations x_m are plotted in the middle and right panels in c–e.