Both the training and test sets, each containing only molecules composed of carbon, hydrogen, and oxygen, exercise a total of 18 SMIRNOFF LJ parameters (9 different SMIRKS parameter types with one and per SMIRKS). These LJ parameters in OpenFF 1.0.0 have not been optimized since their inception in the first SMIRNOFF format force field , and are taken chiefly from AMBER parm94 , with the exception of the hydroxyl hydrogen parameter discussed below. Of these parameters, 12 were optimized, with the remaining 6 held constant at their initial OpenFF 1.0.0 values. The parameters held constant (all for hydrogens) were not optimized because either the parameter correspond to a specific context that was not sufficiently constrained by the training data set or, in the case of [#1:1]-[#8] (hydroxyl hydrogen), the OpenFF 1.0.0 value is explicitly set to a very small nonzero value ( = 5.27 × 10 -5 ) and not reoptimized. This is a slight modification of the AMBER hydroxyl hydrogen parameter (HO, = 0) to avoid unphysical effects caused by the AMBER parameterization . Here each parameter is uniquely identified by a SMIRKS pattern which encodes the chemical environment to which the parameter will be applied . These parameters, along with brief descriptions, are listed in Table . Hydrogen attached to generic oxygen . Each of these atom types has a and parameter that describe the Lennard-Jones interactions; with 6 SMIRKS types included in the optimization, 12 Lennard-Jones parameters were optimized. In the "illustration" figures, any atomic index including a '*' is a wildcard, representing any atom, or group of atoms.