where E is the potential energy, β ≡ 1 k B T the inverse temperature, k B Boltzmann's constant, and T the temperature. ∆E + and ∆E -are calculated by making two perturbations to the surface area S ≡ L x L y , by ∆S = ±0.0005S as suggested in Ref. 27. In each perturbation, the x and y dimensions of the simulation box are scaled proportionally, while the z dimension is scaled in the opposite direction to keep the total volume constant. The scaling operation is also applied to the molecular centroids, and the molecules are rigidly translated without modifying the molecular geometry. The ensemble averages in the formula are evaluated as the arithmetic average over trajectory frames.