Using OPC water models for RBFE calculations

Dear all,

I have been trying to use other water models such as OPC (a 4 point model).

I thought that intuitively it should be as simple as:

rbfe_settings = RelativeHybridTopologyProtocol.default_settings()

rbfe_settings.forcefield_settings.forcefields=["amber/RNA.OL3.xml","amber/opc_standard.xml"]
rbfe_settings.solvation_settings.solvent_model="opc"

However, I get an error that says:

File /data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:384, in BaseModel.__setattr__(self, name, value)
    [382](https://vscode-remote+ssh-002dremote-002blinux.vscode-resource.vscode-cdn.net/data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:382) value, error_ = known_field.validate(value, dict_without_original_value, loc=name, cls=self.__class__)
    [383](https://vscode-remote+ssh-002dremote-002blinux.vscode-resource.vscode-cdn.net/data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:383) if error_:
--> [384](https://vscode-remote+ssh-002dremote-002blinux.vscode-resource.vscode-cdn.net/data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:384)     raise ValidationError([error_], self.__class__)
    [385](https://vscode-remote+ssh-002dremote-002blinux.vscode-resource.vscode-cdn.net/data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:385) else:
    [386](https://vscode-remote+ssh-002dremote-002blinux.vscode-resource.vscode-cdn.net/data/sw/mamba/envs/openfe_env/lib/python3.12/site-packages/pydantic/v1/main.py:386)     new_values[name] = value

ValidationError: 1 validation error for OpenMMSolvationSettings
solvent_model
  unexpected value; permitted: 'tip3p', 'spce', 'tip4pew', 'tip5p' (type=value_error.const; given=opc; permitted=('tip3p', 'spce', 'tip4pew', 'tip5p'))

I tried to go deep in the gufe rabbit hole yet it was a bit unclear.

I thought I could just set up another 4 point model such as tip4pew and run the system with it, as the opc parameters should override the tip4pew setup. Doing this, I could write the transformations to disk. However, when running the json files with openfe quickrun, I got the following output:

Done with all simulations! Analyzing the results....
Here is the result:
        dG = None ± None

Error: The protocol unit 'LIG1 to LIG2 repeat 0 generation 0' failed with the error message:
ValueError: Residue template U with the same override level 0 already exists.

Details provided in output.

For reference, the same simulation runs properly when using the default force field parameters.

Thanks,
O.