We calculated the free energy difference between the stable and metastable forms from potentials of mean force (PMF) obtained with umbrella sampling . To do this, we used harmonic potentials of spring constant spring constant 200 kJ mol -1 to restrain the central double bond torsion angle in a series of 1µs MD simulation windows. The torsion value at each window center ranges from -1.9 to 2.1 radian, with 0.1 radian spacing. The central torsion angle is defined here as 43-15-14-29, where 43 and 29 are hydrogens attached to C18 and C2 in Fig , respectively. This range spans the metastable form, the stable form, and the barrier between them. Umbrella sampling simulations were set up using the open-source library PLUMED , and the PMF along the torsion was obtained by Weighted Histogram Analysis Method (WHAM) . From the PMF F (x), which is essentially the conditional probability distribution expressed as a free energy profile, the free energy difference between the metastable form and the stable form was calculated by integrating across the corresponding ranges of the torsion values: x) dx. Here the s and m subscripts indicate the sampling windows that span the stable and metastable forms, respectively.