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We describe the structure and optimization of the Open Force Field 1.0.0 small molecule force field, code-named Parsley. Parsley uses the SMIRKS-native Open Force Field (SMIRNOFF) parameter assignment formalism in which parameter types are assigned directly by chemical perception, in contrast to traditional atom type-based approaches. This method provides a natural means to incorporate increasingly diverse chemistry without needlessly increasing force field complexity. In this work, we present essentially a full optimization of the valence parameters in the force field. The optimization was carried out with the ForceBalance tool and was informed by reference quantum chemical data that include torsion potential energy profiles, optimized gas-phase structures, and vibrational frequencies. These data were computed and are maintained with QCArchive, an open-source and freely available distributed computing and database software ecosystem. Tests of the resulting force field against compounds and data types outside the training set show improvements in optimized geometries and conformational energetics and demonstrate that Parsley's accuracy for liquid properties is similar to that of other general force fields.
MKG has an equity interest in and is a cofounder and scientiﬁc advisor of VeraChem LLC.
DLM is a current member of the Scientiﬁc Advisory Board of OpenEye Scientiﬁc Software and an Open
548 Science Fellow for Silicon Therapeutics.
MRS is an Open Science Fellow for Silicon Therapeutics.
JDC is a current member of the Scientiﬁc Advisory Board of OpenEye Scientiﬁc Software and scientiﬁc
544 consultant to Foresite Labs.