Molecular Dynamics

Overview

Molecular dynamics (MD) simulation computationally models the time evolution of molecular systems by integrating Newton's equations of motion. This skill covers two complementary tools:

• OpenMM (https://openmm.org/): High-performance MD simulation engine with GPU support, Python API, and flexible force field support
• MDAnalysis (https://mdanalysis.org/): Python library for reading, writing, and analyzing MD trajectories from all major simulation packages

Installation (verified versions: OpenMM 8.x, MDAnalysis 2.x):

# uv (preferred) — both ship binary wheels for arm64 macOS, no conda needed
uv add openmm "mdanalysis>=2.9" pdbfixer
# pdbfixer is not on PyPI for all platforms; if the wheel is unavailable:
uv pip install "pdbfixer @ git+https://github.com/openmm/pdbfixer.git"

# conda-forge alternative (pulls CUDA builds on Linux):
# conda install -c conda-forge openmm mdanalysis pdbfixer nglview

Note: on Apple Silicon there is no CUDA; OpenMM falls back to the CPU (and, on macOS, OpenCL/Metal) platforms — see the platform-selection block below.

When to Use This Skill

Use molecular dynamics when:

• Protein stability analysis: How does a mutation affect protein dynamics?
• Drug binding simulations: Characterize binding mode and residence time of a ligand
• Conformational sampling: Explore protein flexibility and conformational changes
• Protein-protein interaction: Model interface dynamics and binding energetics
• RMSD/RMSF analysis: Quantify structural fluctuations from a reference structure
• Free energy estimation: Compute binding free energy or conformational free energy
• Membrane simulations: Model proteins in lipid bilayers
• Intrinsically disordered proteins: Study IDR conformational ensembles

Core Workflow: OpenMM Simulation

1. System Preparation

from openmm.app import *
from openmm import *
from openmm.unit import *
import sys

def prepare_system_from_pdb(pdb_file, forcefield_name="amber14-all.xml",
                              water_model="amber14/tip3pfb.xml"):
    """
    Prepare an OpenMM system from a PDB file.

    Args:
        pdb_file: Path to cleaned PDB file (use PDBFixer for raw PDB files)
        forcefield_name: Force field XML file
        water_model: Water model XML file

    Returns:
        pdb, forcefield, system, topology
    """
    # Load PDB
    pdb = PDBFile(pdb_file)

    # Load force field
    forcefield = ForceField(forcefield_name, water_model)

    # Add hydrogens and solvate
    modeller = Modeller(pdb.topology, pdb.positions)
    modeller.addHydrogens(forcefield)

    # Add solvent box (10 Å padding, 150 mM NaCl)
    modeller.addSolvent(
        forcefield,
        model='tip3p',
        padding=10*angstroms,
        ionicStrength=0.15*molar
    )

    print(f"System: {modeller.topology.getNumAtoms()} atoms, "
          f"{modeller.topology.getNumResidues()} residues")

    # Create system
    system = forcefield.createSystem(
        modeller.topology,
        nonbondedMethod=PME,         # Particle Mesh Ewald for long-range electrostatics
        nonbondedCutoff=1.0*nanometer,
        constraints=HBonds,           # Constrain hydrogen bonds (allows 2 fs timestep)
        rigidWater=True,
        ewaldErrorTolerance=0.0005
    )

    return modeller, system

2. Energy Minimization

from openmm.app import *
from openmm import *
from openmm.unit import *

def minimize_energy(modeller, system, output_pdb="minimized.pdb",
                     max_iterations=1000, tolerance=10.0):
    """
    Energy minimize the system to remove steric clashes.

    Args:
        modeller: Modeller object with topology and positions
        system: OpenMM System
        output_pdb: Path to save minimized structure
        max_iterations: Maximum minimization steps
        tolerance: Convergence criterion in kJ/mol/nm

    Returns:
        simulation object with minimized positions
    """
    # Set up integrator (doesn't matter for minimization)
    integrator = LangevinMiddleIntegrator(300*kelvin, 1/picosecond, 0.004*picoseconds)

    # Create simulation
    # Use GPU if available (CUDA or OpenCL), fall back to CPU
    try:
        platform = Platform.getPlatformByName('CUDA')
        properties = {'DeviceIndex': '0', 'Precision': 'mixed'}
    except Exception:
        try:
            platform = Platform.getPlatformByName('OpenCL')
            properties = {}
        except Exception:
            platform = Platform.getPlatformByName('CPU')
            properties = {}

    simulation = Simulation(
        modeller.topology, system, integrator,
        platform, properties
    )
    simulation.context.setPositions(modeller.positions)

    # Check initial energy
    state = simulation.context.getState(getEnergy=True)
    print(f"Initial energy: {state.getPotentialEnergy()}")

    # Minimize
    simulation.minimizeEnergy(
        tolerance=tolerance*kilojoules_per_mole/nanometer,
        maxIterations=max_iterations
    )

    state = simulation.context.getState(getEnergy=True, getPositions=True)
    print(f"Minimized energy: {state.getPotentialEnergy()}")

    # Save minimized structure
    with open(output_pdb, 'w') as f:
        PDBFile.writeFile(simulation.topology, state.getPositions(), f)

    return simulation

3. NVT Equilibration

from openmm.app import *
from openmm import *
from openmm.unit import *

def run_nvt_equilibration(simulation, n_steps=50000, temperature=300,
                            report_interval=1000, output_prefix="nvt"):
    """
    NVT equilibration: constant N, V, T.
    Equilibrate velocities to target temperature.

    Args:
        simulation: OpenMM Simulation (after minimization)
        n_steps: Number of MD steps (50000 × 2fs = 100 ps)
        temperature: Temperature in Kelvin
        report_interval: Steps between data reports
        output_prefix: File prefix for trajectory and log
    """
    # Add position restraints for backbone during NVT
    # (Optional: restraint heavy atoms)

    # Set temperature
    simulation.context.setVelocitiesToTemperature(temperature*kelvin)

    # Add reporters
    simulation.reporters = []

    # Log file
    simulation.reporters.append(
        StateDataReporter(
            f"{output_prefix}_log.txt",
            report_interval,
            step=True,
            potentialEnergy=True,
            kineticEnergy=True,
            temperature=True,
            volume=True,
            speed=True
        )
    )

    # DCD trajectory (compact binary format)
    simulation.reporters.append(
        DCDReporter(f"{output_prefix}_traj.dcd", report_interval)
    )

    print(f"Running NVT equilibration: {n_steps} steps ({n_steps*2/1000:.1f} ps)")
    simulation.step(n_steps)
    print("NVT equilibration complete")

    return simulation

4. NPT Equilibration and Production

def run_npt_production(simulation, n_steps=500000, temperature=300, pressure=1.0,
                        report_interval=5000, output_prefix="npt"):
    """
    NPT production run: constant N, P, T.

    Args:
        n_steps: Production steps (500000 × 2fs = 1 ns)
        temperature: Temperature in Kelvin
        pressure: Pressure in bar
        report_interval: Steps between reports
    """
    # Add Monte Carlo barostat for pressure control
    system = simulation.context.getSystem()
    system.addForce(MonteCarloBarostat(pressure*bar, temperature*kelvin, 25))
    simulation.context.reinitialize(preserveState=True)

    # Update reporters
    simulation.reporters = []
    simulation.reporters.append(
        StateDataReporter(
            f"{output_prefix}_log.txt",
            report_interval,
            step=True,
            potentialEnergy=True,
            temperature=True,
            density=True,
            speed=True
        )
    )
    simulation.reporters.append(
        DCDReporter(f"{output_prefix}_traj.dcd", report_interval)
    )

    # Save checkpoints
    simulation.reporters.append(
        CheckpointReporter(f"{output_prefix}_checkpoint.chk", 50000)
    )

    print(f"Running NPT production: {n_steps} steps ({n_steps*2/1000000:.2f} ns)")
    simulation.step(n_steps)
    print("Production MD complete")
    return simulation

Trajectory Analysis with MDAnalysis

1. Load Trajectory

import MDAnalysis as mda
from MDAnalysis.analysis import rms, align, contacts
import numpy as np
import matplotlib.pyplot as plt

def load_trajectory(topology_file, trajectory_file):
    """
    Load an MD trajectory with MDAnalysis.

    Args:
        topology_file: PDB, PSF, or other topology file
        trajectory_file: DCD, XTC, TRR, or other trajectory
    """
    u = mda.Universe(topology_file, trajectory_file)
    print(f"Universe: {u.atoms.n_atoms} atoms, {u.trajectory.n_frames} frames")
    print(f"Time range: 0 to {u.trajectory.totaltime:.0f} ps")
    return u

2. RMSD Analysis

def compute_rmsd(u, selection="backbone", reference_frame=0):
    """
    Compute RMSD of selected atoms relative to reference frame.

    Args:
        u: MDAnalysis Universe
        selection: Atom selection string (MDAnalysis syntax)
        reference_frame: Frame index for reference structure

    Returns:
        numpy array of (time, rmsd) values
    """
    # Pre-align the trajectory (rewrites coordinates in memory) so that any
    # downstream per-atom analysis sees superimposed frames.
    aligner = align.AlignTraj(u, u, select=selection, in_memory=True)
    aligner.run()

    # rms.RMSD does its own optimal superposition against ref_frame, so the
    # RMSD numbers below do not depend on the AlignTraj step above.
    R = rms.RMSD(u, select=selection, ref_frame=reference_frame)
    R.run()

    rmsd_data = R.results.rmsd  # columns: frame, time, RMSD
    return rmsd_data

def plot_rmsd(rmsd_data, title="RMSD over time", output_file="rmsd.png"):
    """Plot RMSD over simulation time."""
    fig, ax = plt.subplots(figsize=(10, 4))
    ax.plot(rmsd_data[:, 1] / 1000, rmsd_data[:, 2], 'b-', linewidth=0.5)
    ax.set_xlabel("Time (ns)")
    ax.set_ylabel("RMSD (Å)")
    ax.set_title(title)
    ax.axhline(rmsd_data[:, 2].mean(), color='r', linestyle='--',
               label=f'Mean: {rmsd_data[:, 2].mean():.2f} Å')
    ax.legend()
    plt.tight_layout()
    plt.savefig(output_file, dpi=150)
    return fig

3. RMSF Analysis (Per-Residue Flexibility)

def compute_rmsf(u, selection="backbone", start_frame=0):
    """
    Compute per-residue RMSF (flexibility).

    Returns:
        resids, rmsf_values arrays
    """
    # Select atoms
    atoms = u.select_atoms(selection)

    # Compute RMSF
    R = rms.RMSF(atoms)
    R.run(start=start_frame)

    # Average by residue
    resids = []
    rmsf_per_res = []
    for res in u.select_atoms(selection).residues:
        res_atoms = res.atoms.intersection(atoms)
        if len(res_atoms) > 0:
            resids.append(res.resid)
            rmsf_per_res.append(R.results.rmsf[res_atoms.indices].mean())

    return np.array(resids), np.array(rmsf_per_res)

4. Protein-Ligand Contacts

from MDAnalysis.lib import distances as mda_distances

def analyze_contacts(u, protein_sel="protein", ligand_sel="resname LIG",
                      radius=4.5, start_frame=0):
    """
    Track which protein residues are in contact with the ligand per frame.

    Args:
        radius: Contact distance cutoff in Angstroms

    Returns:
        list of sets, one per analyzed frame, of contacting protein resids
    """
    protein = u.select_atoms(protein_sel)
    ligand = u.select_atoms(ligand_sel)

    contact_frames = []
    for ts in u.trajectory[start_frame:]:
        # Pairwise protein-ligand distance matrix (PBC-aware via box=u.dimensions),
        # then threshold into a boolean contact matrix.
        # NOTE: contacts.contact_matrix(d, radius) takes a *distance matrix* d,
        # not raw coordinates — passing positions directly is a common bug.
        dmat = mda_distances.distance_array(
            protein.positions, ligand.positions, box=u.dimensions
        )
        contact_mask = contacts.contact_matrix(dmat, radius)  # shape (n_prot, n_lig)

        # A protein atom is in contact if it is close to any ligand atom.
        contacting_atoms = protein.atoms[contact_mask.any(axis=1)]
        contact_frames.append(set(contacting_atoms.resids))

    return contact_frames

Force Field Selection Guide

System	Recommended Force Field	Water Model
Standard proteins	AMBER14 (amber14-all.xml)	TIP3P-FB
Proteins + small molecules	AMBER14 + GAFF2	TIP3P-FB
Membrane proteins	CHARMM36m	TIP3P
Nucleic acids	AMBER99-bsc1 or AMBER14	TIP3P
Disordered proteins	ff19SB or CHARMM36m	TIP3P

System Preparation Tools

PDBFixer (for raw PDB files)

from pdbfixer import PDBFixer
from openmm.app import PDBFile

def fix_pdb(input_pdb, output_pdb, ph=7.0):
    """Fix common PDB issues: missing residues, atoms, add H, standardize."""
    fixer = PDBFixer(filename=input_pdb)
    fixer.findMissingResidues()
    fixer.findNonstandardResidues()
    fixer.replaceNonstandardResidues()
    fixer.removeHeterogens(True)    # Remove water/ligands
    fixer.findMissingAtoms()
    fixer.addMissingAtoms()
    fixer.addMissingHydrogens(ph)

    with open(output_pdb, 'w') as f:
        PDBFile.writeFile(fixer.topology, fixer.positions, f)

    return output_pdb

Small-Molecule Parameterization (via OpenFF Toolkit)

# uv add openff-toolkit openff-interchange
# openff-2.2.0.offxml is OpenFF "Sage" — a SMIRNOFF force field, NOT GAFF2.
# (For actual GAFF2, parameterize with AmberTools antechamber/ACPYPE instead.)
from openff.toolkit import Molecule, ForceField as OFFForceField

def parameterize_ligand(smiles, ff_name="openff-2.2.0.offxml"):
    """Generate OpenFF (Sage) parameters for a small molecule as an Interchange."""
    mol = Molecule.from_smiles(smiles)
    mol.generate_conformers(n_conformers=1)
    mol.assign_partial_charges("am1bcc")  # AM1-BCC charges, OpenFF default

    off_ff = OFFForceField(ff_name)
    interchange = off_ff.create_interchange(mol.to_topology())
    return interchange  # -> interchange.to_openmm() for an OpenMM System

Best Practices

• Always minimize before MD: Raw PDB structures have steric clashes
• Equilibrate before production: NVT (50–100 ps) → NPT (100–500 ps) → Production
• Use GPU: Simulations are 10–100× faster on GPU (CUDA/OpenCL)
• 2 fs timestep with HBonds constraints: Standard; use 4 fs with HMR (hydrogen mass repartitioning)
• Analyze only equilibrated trajectory: Discard first 20–50% as equilibration
• Save checkpoints: MD runs can fail; checkpoints allow restart
• Periodic boundary conditions: Required for solvated systems
• PME for electrostatics: More accurate than cutoff methods for charged systems

Additional Resources

• OpenMM documentation: https://docs.openmm.org/latest/userguide/
• MDAnalysis user guide: https://docs.mdanalysis.org/
• GROMACS (alternative MD engine): https://manual.gromacs.org/
• NAMD (alternative): https://www.ks.uiuc.edu/Research/namd/
• CHARMM-GUI (web-based system builder): https://charmm-gui.org/
• AmberTools (free Amber tools): https://ambermd.org/AmberTools.php
• OpenMM paper: Eastman P et al. (2017) PLOS Computational Biology 13(7):e1005659. PMID: 28746339 (DOI: 10.1371/journal.pcbi.1005659)
• MDAnalysis paper: Michaud-Agrawal N et al. (2011) J Computational Chemistry. PMID: 21500218