The following examples are available in the openforcefield toolkit repository:
SMIRNOFF_simulation - simulation of a molecule in the gas phase with the SMIRNOFF forcefield format
forcefield_modification - modify forcefield parameters and evaluate how system energy changes
using_smirnoff_in_amber_or_gromacs - convert a System generated with the Open Forcefield Toolkit, which can be simulated natively with OpenMM, into AMBER prmtop/inpcrd and GROMACS top/gro input files through the ParmEd library.
inspect_assigned_parameters - check which parameters are used in which molecules and generate parameter usage statistics.
using_smirnoff_with_amber_protein_forcefield - use SMIRNOFF parameters for small molecules in combination with more conventional force fields for proteins and other components of your system (using ParmEd to combine parameterized structures)