The reference data used to parameterize AMBER-FB18 is comprised of QM single-point energies, gradients and vibrational modes, largely following the procedure described for canonical amino acids in parameterizing AMBER-FB15. The single-point energies and gradients were computed for constrained energy-minimized structures where constraints were varied over 2-D grids of main chain or side chain dihedral angles. The TorsionDrive 67 procedure was employed to generate the grids of constrained optimized structures using a wavefront propagation method as illustrated in Figure . The procedure starts with a userspecified grid of dihedral angle constraints and one or more initial structures. In the first iteration, the initial structures are energy-minimized with the dihedral angle constraints set to the nearest grid point. Next, constrained energy minimizations are started from the minimized structures with new target constraint values set to the neighboring grid points (4 for a 2-dimensional scan), and the cycle is repeated. As the iterations proceed, multiple minimizations may be targeted at the same grid point starting from different neighboring points, or new minimizations may be targeted at a grid point that already has results from a previous iteration. When this occurs, the energies of the newly finished minimizations are compared with any previous results at that grid point, and new calculations are launched in the next iteration only if a new lowest-energy structure is found. Thus, at the end of the TorsionDrive procedure, the dihedral grid has an energy-minimized structure at each point with the property that no new constrained minimizations started from neighboring grid points could further lower the energy. While TorsionDrive is computationally more expensive than running the constrained minimizations separately, it has the advantage of producing a maximally continuous potential energy surface even when there are multiple local minima in the orthogonal degrees of freedom. Iteration 0: (not shown) The user-provided initial structure is energy-minimized with dihedral constraints set to the nearest grid point at (120 • , 0 • ).