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 as of 2026-09: OpenMM 8.6.1, MDAnalysis 2.10, pdbfixer 1.12):
bash
# uv (preferred) — all three publish binary wheels on PyPI, no conda needed
uv add openmm "mdanalysis>=2.10" pdbfixer
# If a pdbfixer wheel is unavailable for your platform, install from source:
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
Does NOT Trigger
| Scenario | Use Instead | |
|---|
| Predicting an initial protein–ligand binding pose (no dynamics) | alterlab-diffdock | |
| Quantum chemistry, pKa, or cloud AutoDock Vina docking scores | alterlab-rowan | |
| Predicting the protein structure to simulate (no experimental PDB) | alterlab-alphafold or alterlab-boltz | |
| Parameterizing a small-molecule force field only (OpenFF/GAFF), no simulation | alterlab-rdkit for structure prep, then OpenFF (below) | |
Core Workflow: OpenMM Simulation
1. System Preparation
python
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
python
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
"""
# This integrator is reused by the NVT/NPT stages below, so use 2 fs: with only
# HBonds constraints (no hydrogen mass repartitioning) a 4 fs step is unstable.
integrator = LangevinMiddleIntegrator(300*kelvin, 1/picosecond, 0.002*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
python
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
python
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
python
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
python
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)
python
def compute_rmsf(u, selection="backbone", start_frame=0):
"""
Compute per-residue RMSF (flexibility).
Returns:
resids, rmsf_values arrays
"""
atoms = u.select_atoms(selection)
# RMSF does no superposition of its own — align the trajectory to the average
# structure first, otherwise the fluctuations include rigid-body motion.
average = align.AverageStructure(u, u, select=selection, ref_frame=0).run()
align.AlignTraj(u, average.results.universe, select=selection, in_memory=True).run()
R = rms.RMSF(atoms).run(start=start_frame)