In [1]: from openmmforcefields.generators import SMIRNOFFTemplateGenerator

In [2]: from openff.toolkit import Topology

In [3]: import openmm.app

In [4]: topology = Topology.from_pdb("ctr148a-2KO1-model-1.pdb")

In [5]: ff = openmm.app.ForceField()

In [6]: gen = SMIRNOFFTemplateGenerator(molecules=[*topology.molecules], forcefield='openff_no_water_unconstrained-3.0.0-alpha0.offxml')

In [7]: ff.registerTemplateGenerator(gen.generator)

In [8]: ff.createSystem(topology.to_openmm())
Did not recognize residue MET; did you forget to call .add_molecules() to add it?
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[8], line 1
----> 1 ff.createSystem(topology.to_openmm())

File ~/mamba/envs/openmmforcefields/lib/python3.13/site-packages/openmm/app/forcefield.py:1266, in ForceField.createSystem(self, topology, nonbondedMethod, nonbondedCutoff, constraints, rigidWater, removeCMMotion, hydrogenMass, residueTemplates, ignoreExternalBonds, switchDistance, flexibleConstraints, drudeMass, **args)
   1262 rigidResidue = [False]*topology.getNumResidues()
   1264 # Find the template matching each residue and assign atom types.
-> 1266 templateForResidue = self._matchAllResiduesToTemplates(data, topology, residueTemplates, ignoreExternalBonds)
   1267 for res in topology.residues():
   1268     if res.name == 'HOH':
   1269         # Determine whether this should be a rigid water.

File ~/mamba/envs/openmmforcefields/lib/python3.13/site-packages/openmm/app/forcefield.py:1486, in ForceField._matchAllResiduesToTemplates(self, data, topology, residueTemplates, ignoreExternalBonds, ignoreExtraParticles, recordParameters)
   1484                 break
   1485 if matches is None:
-> 1486     raise ValueError('No template found for residue %d (%s).  %s  For more information, see https://github.com/openmm/openmm/wiki/Frequently-Asked-Questions#template' % (res.index, res.name, _findMatchErrors(self, res)))
   1487 else:
   1488     if recordParameters:

ValueError: No template found for residue 0 (MET).  The force field contains no residue templates.  For more information, see https://github.com/openmm/openmm/wiki/Frequently-Asked-Questions#template

In [1]: from openff.toolkit import Topology, ForceField

In [2]: topology = Topology.from_pdb("ctr148a-2KO1-model-1.pdb")

In [3]: ForceField('openff_no_water_unconstrained-3.0.0-alpha0.offxml').create_openmm_system(topology)
[13:37:59] WARNING: Proton(s) added/removed

Out[3]: <openmm.openmm.System; proxy of <Swig Object of type 'OpenMM::System *' at 0x107a2a870> >