The accuracy of classical MD depends on the accuracy of the force field used. To our knowledge, no specialized force field exists for alkene motors, except one reparametrized from OPLS for classical MD simulation of photoisomerization, rather than THI . In addition, our initial tests of GAFF2 force field yielded problematic results for these molecules, as detailed below. We therefore parameterized a specialized force field for each motor molecule by fitting to its unique QM potential energy surfaces and gradients, as detailed below.