How can I define in which directions velocities are generated by a thermostat?

[aldorvasq]

Hello HOOMD users,

I am currently performing some simulations in the NVT ensemble that involve velocity generation via a thermostat (Langevin) in a specific direction only, say the y-direction, while leaving the other components (x and z) unconstrained. I have looked through the documentation and, to my best knowledge, I could not find anything that could do that. I thought of employing a filter of type hoomd.filter.CustomFilter() to the thermostat but it is not obvious to me how that could be implemented for this scenario. Is there an easier way to implement the velocity generation in a specific direction?

Thank you,

Aldo V.

[joaander]

Langevin operates by applying forces to the particles. The guide "How to apply arbitrary forces in MD" describes the ways in which you can implement your custom force computation.

[aldorvasq]

Thank you @joaander for the prompt response,

From what I understood from your suggestion, should the proper implementation of what I am looking for then be to use hoomd.md.methods.ConstantVolume(thermostat=None) for the time integration and then remake the Langevin thermostat (excluding the forces by non-bonded and bonded potentials) from scratch as an arbitrary force where I redefine the random forces (in the specific direction I want) and the damping term?. I am not really looking to add additional forces to the system but to restrain which direction the forces (and the damping term) are applied by hoomd.md.methods.Langevin(). Something similar to compute temp/partial in LAMMPS (https://docs.lammps.org/compute_temp_partial.html).

[joaander]

Your assessment of the HOOMD-blue documentation was correct. HOOMD-blue does not provide the option you describe. Therefore, the behavior your desire is custom. You can achieve your desired custom behavior with a custom force or by modifying the C++ class TwoStepLangevin.