Polydisperse LJ Polymer Generator

A tool for generating polydisperse polymer systems for LAMMPS simulations.

Workflow

Data Flow

graph TD
    A[input_config.yaml] --> B[Generate System]
    B --> C[Chain Coordinates]
    B --> D[Topology File]
    C --> E[Packmol]
    E --> F[Packed System]
    D --> G[LAMMPS Data]
    F --> G
    D --> H[LAMMPS Input]
    G --> I[LAMMPS Simulation]
    H --> I
    I --> J[Trajectory]
    I --> K[Thermodynamics]

    subgraph Generation
        B
        C
        D
    end

    subgraph Packing
        E
        F
    end

    subgraph Simulation
        G
        H
        I
        J
        K
    end

    style A fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#bbf,stroke:#333,stroke-width:2px
    style E fill:#bfb,stroke:#333,stroke-width:2px
    style I fill:#fbf,stroke:#333,stroke-width:2px

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Command Flow

sequenceDiagram
    participant U as User
    participant F as flow
    participant G as generate
    participant P as packmol
    participant L as lammps
    participant S as simulation

    U->>F: polydispers flow
    activate F
    F->>G: Generate System?
    G-->>F: Generated Files
    F->>P: Run Packmol?
    P-->>F: Packed System
    F->>L: Prepare LAMMPS?
    L-->>F: LAMMPS Files
    F->>S: Run Simulation?
    S-->>F: Results
    deactivate F

    Note over U,S: Alternative: Manual Steps
    U->>G: polydispers generate
    G-->>U: Generated Files
    U->>P: packmol < input
    P-->>U: Packed System
    U->>L: polydispers lammps
    L-->>U: LAMMPS Files
    U->>S: lmp -in script.in
    S-->>U: Results

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Features

• Generate polymer systems with specified:
  • Number of chains
  • Number-average molecular weight (Mn)
  • Polydispersity index (PDI)
  • Box size
  • Bond length
  • Bead radius
• Support for different bead types (A-B polymers)
• Automatic LAMMPS input file generation
• Integration with Packmol for system packing

Schulz-Zimm Distribution

The tool calculates the Schulz-Zimm distribution parameters based on the target chain length and PDI.

• k parameter: 1 / (PDI - 1)
• theta parameter: target chain length / (k + 1)

The tool prints the distribution statistics, including the mean chain length, PDI, and Schulz-Zimm parameters.

Example of the output

Requirements

• Python 3.11+
• Packmol
• LAMMPS (any variant: lmp, lmp_serial, or lmp_mpi)

Installation

1. Clone the repository:

git clone https://github.com/jkrajniak/polydispers-lj-generator.git
cd polydispers-lj-generator

2. Create and activate a virtual environment:

python -m venv .venv
source .venv/bin/activate  # On Unix/macOS
# or
.venv\Scripts\activate  # On Windows

3. Install the package:

pip install -e .

Usage

Configuration

Create an input configuration file (e.g., input_config.yaml):

num_chains: 10
mn: 1000
pdi: 1.2
box_size: 100
output_dir: ./sys1
seed: 42
polymer:
  bond_length: 0.85
  bead_radius: 1.0
  repeat_unit_topology: AB
  bead_types:
    A:
      mass: 1.0
      type_id: 1
    B:
      mass: 1.0
      type_id: 2

Commands

1. Generate polymer system:

polydispers generate --config input_config.yaml

2. Interactive workflow (recommended):

polydispers flow --config input_config.yaml

This will guide you through:

• System generation
• Packmol packing
• LAMMPS file preparation
• LAMMPS simulation

3. Prepare LAMMPS files separately:

polydispers lammps --topology-file topology.yaml --coordinates coordinates.xyz

Output Files

The tool generates several files in the output directory:

• topology.yaml: System topology description
• chain_*.xyz: Individual chain coordinates
• packmol_input.txt: Packmol input script
• lj.xyz: Packed system coordinates (after running Packmol)
• lj.data: LAMMPS data file
• lj.in: LAMMPS input script
• instructions.sh: Shell script with next steps
• thermo.dat: LAMMPS thermodynamic output (after simulation)
• traj.lammpstrj: LAMMPS trajectory file (after simulation)

LAMMPS Simulation Details

The generated LAMMPS input script includes:

• LJ potential with cutoff 2.5σ
• FENE bonds
• NPT equilibration with Berendsen thermostat
• NVT production run
• Trajectory and thermodynamic output

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

This project is licensed under the MIT License - see the LICENSE file for details.