The first chosen surrogate candidate is the General Amber Force Field (GAFF) as provided in the Amber 2018 suite. Fixed partial charges, atom types and force field parameters have been assigned with the Antechamber and Leap tools. Atomic charges are derived from the default restrained electrostatic potential (RESP) fit at the HF/6-31(d) level of theory. For the purpose of comparison and to test the sensitivity with respect to the choice of fixed point charges, we also used charges derived with the faster Austin Model 1 with bond charge correction (AM1-BCC) scheme. For both cis and trans GPGG, charges are calculated from structures constructed with the amino acid sequence editor of Molden, while the X-ray resolved crystal structure is used for gramicidin. Van der Waals and electrostatic interactions are not truncated in the absence of periodic boundary conditions. Local geometry optimizations are performed using Sander single-core jobs consisting first of 4000 steepest descent steps followed by conjugate gradient optimization until convergence to the default 10 -4 kcal/(mol Å) root-mean-square deviation of the Cartesian elements of the gradient.