Active Paramagnetic Particles Simulation (HOOMD-blue)

Overview

This repository contains a proof-of-concept simulation script for modeling active paramagnetic particles using HOOMD-blue.

The script demonstrates particle dynamics where dipole orientations are fixed in the global reference frame, enabling controlled studies of active matter systems subjected to strong external fields. Particles can still undergo rotational diffusion independently of the dipole orientation via Néel relaxation, introducing rich dynamic behavior arising from the decoupling of mechanical rotation and magnetic moment dynamics.

Requirements

This project depends on a modified version of HOOMD-blue that provides functionality not available in the standard release.

Specifically, the required build includes:

• An ignore_rotations flag in the Dipole pair potential

This feature enables:

• Freezing dipole moments in the global reference frame
• Decoupling particle rotations from dipole orientation

The required HOOMD-blue modifications are maintained in a separate development branch:

• Branch name: dipole-update
• Status: Not yet pushed to the public HOOMD-blue repository

Additional requirements and constraints:

• This is a breaking update: running this script with the stable/latest HOOMD-blue release will result in errors
• The implementation is subject to change as the codebase is still under active development

---

Changes from Vanilla HOOMD-blue

The modified interface introduces an additional dipole configuration layer compared to the standard HOOMD-blue API.

Vanilla version

dipole = hoomd.md.pair.aniso.Dipole(cell, default_r_cut=r_cut)
dipole.params[("A", "A")] = dict(A=40, kappa=0)
dipole.mu["A"] = (0.0, 0.0, 1.0)

Proposed update

dipole = hoomd.md.pair.aniso.Dipole(cell, default_r_cut=r_cut)
dipole.params[("A", "A")] = dict(A=40, kappa=0)
dipole.dipole_params["A"] = dict(mu=(0.0, 0.0, 1.0), ignore_rotations=True)

Summary of change

• mu is moved from dipole.mu[type] → dipole.dipole_params[type]
• Adds ignore_rotations=True to control rotational coupling
• Separates interaction parameters (params) from dipole configuration (dipole_params)
• Enables decoupling of rotational degrees of freedom from dipole orientation

Simulation