To allow efficient construction of complex SMIRKS patterns that make chemical sense, symmetric moves are also sometimes proposed. In symmetric moves, equivalent modifications are proposed simultaneously to both outer atoms in a bond, angle, or torsion. The probability of making a symmetric move is fixed inside SMIRKY. Specifically, the frequency of symmetric bond, angle, and torsion types in smirnoff99Frosst was used to assign the probability of making symmetric moves for each fragment type by category.