We tested this explanation by replacing the LJ potential with the Buckingham potential , which has a less steep and hence more realistic exponential repulsion term. The initial Buckingham parameters were taken from Table of Engler of coworkers and the B cofficients were optimized without restraint. Bond-stretch and angle-bends were allowed to vary, subject to flat bottom restraints that prevented relative changes greater than 10%. Optimization with the Buckingham potential led to improved agreement with QM, as shown in Fig , without abnormally small force constants. Thus, the Buckingham potential may replace LJ potential for simulations of more congested molecules like Motor O. Unfortunately, support for the Buckingham potential in simulation software is currently limited. For example, it is not implemented in AMBER and, although it is implemented in GROMACS, it is not available in the GPU version. This made it impractical for use in simulations of Motor O at the timescale needed for the present study, so no further results are presented for this molecule. Future progress in software infrastructure is needed to enable Buckingham potential simulations at longer timescales.