how to apply custom force?

[Ye Zhang]

I'm using custom force in Hoomd V-3

class MyCustomForce(hoomd.md.force.Custom):
    def __init__(self):
        super().__init__(aniso=True)

    def set_forces(self,timestep):
        with self.cpu_local_force_arrays as arrays:
            arrays.force[:] += -5
custom_force = MyCustomForce()

If I run this:

custom_force.set_forces(sim.timestep)

I get the error:

How can I apply the forces?

Thanks for your help~

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [21], in <cell line: 9>()
      7             arrays.force[:] += -5
      8 custom_force = MyCustomForce()
----> 9 custom_force.set_forces(sim.timestep)

Input In [21], in MyCustomForce.set_forces(self, timestep)
      5 def set_forces(self,timestep):
----> 6     with self.cpu_local_force_arrays as arrays:
      7         arrays.force[:] += -5

File ~/miniconda3/envs/HoomdLatest/lib/python3.10/site-packages/hoomd/md/force.py:177, in Force.cpu_local_force_arrays(self)
    173 if self._in_context_manager:
    174     raise RuntimeError("Cannot enter cpu_local_force_arrays context "
    175                        "manager inside another local_force_arrays "
    176                        "context manager")
--> 177 return hoomd.md.data.ForceLocalAccess(self)

File ~/miniconda3/envs/HoomdLatest/lib/python3.10/site-packages/hoomd/md/data/local_access.py:28, in _ForceLocalAccessBase.__init__(self, force_obj)
     26 super().__init__()
     27 self._force_obj = force_obj
---> 28 self._cpp_obj = self._cpp_cls(force_obj._cpp_obj)

TypeError: __init__(): incompatible constructor arguments. The following argument types are supported:
    1. hoomd._hoomd.LocalForceComputeDataHost(arg0: hoomd._hoomd.ForceCompute)

Invoked with: None
----------

[tom...@umich.edu]

Hi,

Add the custom force to your simulation's integrator like you would any other force object in hoomd, there is no need to explicitly call the `set_forces` method. The `set_forces` method will be called in the simulation loop automatically.

-Tommy

[Ye Zhang]

Thanks for your help~

Maybe it is helpful  adding  this answer to  the document for people new to python~

[Joshua Anderson]

Ye Zhang,

The documentation already clearly and explicitly states that forces in the `hoomd.md.Integrator.forces` list are those that are applied to the simulation. For example:
https://hoomd-blue.readthedocs.io/en/v3.0.1/package-md.html#hoomd.md.Integrator
https://hoomd-blue.readthedocs.io/en/v3.0.1/tutorial/01-Introducing-Molecular-Dynamics/01-Molecular-Dynamics-Simulations.html
------
Joshua A. Anderson, Ph.D.
Research Area Specialist, Chemical Engineering, University of Michigan

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