Feynman-Kac guided protein design using RFdiffusion. This package implements particle filtering to optimize design objectives during the diffusion process.

Disclaimer: We are currently working on making FK-RFdiffusion easily accessible through improved modifiability, documentation, package-managing and Colab notebooks. For now, follow the instructions below to install FK-RFdiffusion locally by cloning the repo.

FK-RFdiffusion extends RFdiffusion with Feynman-Kac particle filtering for guided protein design. Instead of blind sampling, it guides the generative process toward desired properties.

FK-RFdiffusion is built as a wrapper around RFdiffusion. It also uses ProteinMPNN in its reward function. We therefore use have RFdiffusion and ProteinMPNN as submodules.

To install with submodules, use the --recursive. This will clone RFdiffusion and ProteinMPNN into externals.

git clone --recursive https://github.com/ErikHartman/fk-rfdiffusion.git
cd fk-rfdiffusion

If you already cloned without --recursive, initialize submodules:

git submodule update --init --recursive

FK-RFdiffusion requires a small patch to RFdiffusion to enable runtime control of diffusion timesteps:

cd externals/RFdiffusion
git apply ../../patches/rfdiffusion_timestep_override.patch
cd ../..

This patch preserves your runtime diffuser.T configuration when loading model checkpoints, allowing you to control the number of diffusion steps (e.g., num_diffusion_timesteps=80) without retraining the model.

First, follow the RFdiffusion installation instructions to set up the base environment. This includes creating a conda environment and setting up weights. If you can call the standard RFdiffusion run_inference.py script, you can continue.

We have extra dependencies for the reward functions. PyDSSP is used for secondary structure computation and biopython for sequence-based features.

pip install pydssp biopython

Install pyrosetta according to the documentation.

from fk_rfdiffusion.run_inference_guided import run_feynman_kac_design

run_feynman_kac_design(
    contigs=["A1-50/0 20"],           # Target residues A1-50, then 20-residue binder
    target_structure="target.pdb",     # Your target protein
    reward_function="interface_dG",    # Optimize binding energy
    num_designs=10,
    n_particles=20,                    # Number of parallel particles
    resampling_frequency=5,            # Resample every 5 steps
    guidance_start_timestep=30,        # Start guiding at timestep 30
    output_prefix="./designs/binder"
)

run_feynman_kac_design(
    contigs=["50"],                    # Design a 50-residue protein
    reward_function="alpha_helix_ss",  # Favor alpha helical structure
    num_designs=5,
    n_particles=15,
    output_prefix="./designs/helix"
)

• interface_dG - Binding energy (lower is better)
• alpha_helix_ss - Alpha helix secondary structure content
• beta_sheet_ss - Beta sheet secondary structure content
• loop_ss - Loop/coil secondary structure content
• sequence_hydrophobic - Hydrophobic residue content
• sequence_charged_positive - Positive charge content
• sequence_charged_negative - Negative charge content

You can also pass a custom_reward_fn. More documentation soon!

See fk_rfdiffusion/feynman_kac/reward/configs/presets.yaml for full configuration options.

• n_particles - Number of parallel particles (more = better exploration, slower)
• resampling_frequency - How often to resample particles (lower = more guidance)
• guidance_start_timestep - When to start applying guidance (try 20-50)
• potential_mode - How to compute weights:
  • "difference" (default) - Based on reward improvement
  • "immediate" - Based on current reward
  • "sum" - Based on cumulative future reward
  • "max" - Based on maximum future reward
• tau - Temperature parameter for particle selection (auto-set per reward if None)

If you use this code, please cite:

• The RFdiffusion paper
• The ProteinMPNN paper
• This preprint:

@misc{RFD-FK,
  doi = {10.48550/ARXIV.2511.09216},
  url = {https://arxiv.org/abs/2511.09216},
  author = {Hartman,  Erik and Wallin,  Jonas and Malmström,  Johan and Olsson,  Jimmy},
  title = {Controllable protein design through Feynman-Kac steering},
  publisher = {arXiv},
  year = {2025},
  copyright = {arXiv.org perpetual,  non-exclusive license}
}

MIT