Force Fields — Molecular Mechanics for MD Simulations

Classical force fields define the potential energy of a molecular system as a sum of bonded and non-bonded terms. The parameters (k, r0, θ0, ε, σ, q) define how molecules move and interact. Python-first stack: OpenMM (engine) + OpenFF toolkit (SMIRNOFF small molecule parameterization).

When to Use This Skill

• Setting up MD simulations with AMBER, CHARMM, or OpenFF force fields
• Parameterizing drug-like small molecules (GAFF2, SMIRNOFF Sage)
• Running energy minimization and MD with OpenMM
• Assigning partial charges (AM1-BCC, RESP)
• Understanding energy terms: bonds, angles, torsions, vdW, electrostatics
• Choosing water model (TIP3P, OPC, TIP4P-Ew)
• Analyzing energy decomposition by force group

Quick Start

# Protein-ligand simulation with OpenMM + OpenFF (SMIRNOFF Sage)
from openff.toolkit import Molecule, ForceField
from openff.toolkit.utils.exceptions import ParameterLookupError
from openmmforcefields.generators import SystemGenerator
import openmm.app as app
import openmm as mm
import openmm.unit as unit

# 1. Load protein topology
pdb = app.PDBFile('protein.pdb')

# 2. Parameterize ligand with OpenFF Sage
ligand = Molecule.from_smiles('c1ccc(cc1)CN')
ligand.generate_conformers(n_conformers=1)

# 3. Build system
system_generator = SystemGenerator(
    forcefields=['amber/ff14SB.xml', 'amber/tip3p_standard.xml'],
    small_molecule_forcefield='openff-2.2.0',
    molecules=[ligand],
    forcefield_kwargs={'nonbondedMethod': app.PME, 'constraints': app.HBonds},
)
system = system_generator.create_system(pdb.topology, molecules=[ligand])

# 4. Run with Langevin integrator
integrator = mm.LangevinMiddleIntegrator(
    300 * unit.kelvin, 1.0 / unit.picosecond, 2.0 * unit.femtoseconds
)
simulation = app.Simulation(pdb.topology, system, integrator)
simulation.context.setPositions(pdb.positions)
simulation.minimizeEnergy(maxIterations=500)
simulation.reporters.append(app.DCDReporter('traj.dcd', 1000))
simulation.step(50000)  # 100 ps

Router — What to Read

Task	Reference
Force field theory: energy terms, AMBER/CHARMM/OPLS/GROMOS families	references/ff-fundamentals.md
OpenMM: System, Simulation, integrators, reporters, NPT, restart	references/openmm-basics.md
OpenFF SMIRNOFF toolkit: Molecule, parameterization, Sage 2.2	references/openff-smirnoff.md
GAFF2/antechamber, acpype, CGenFF, AM1-BCC, RESP charges	references/parameterization.md
Energy decomposition, PME, cutoffs, water models, troubleshooting	references/energy-analysis.md

Force Field Families at a Glance

Family	Protein FF	Small Molecule FF	Engine
AMBER	ff14SB, ff19SB	GAFF2	OpenMM, AMBER
CHARMM	CHARMM36m	CGenFF	OpenMM, NAMD, GROMACS
OPLS	OPLS-AA/M	OPLS3e	GROMACS, Schrödinger
OpenFF	—	Sage 2.2, Parsley	OpenMM
GROMOS	54A7	GROMOS-compat	GROMACS (united-atom)

Installation

# OpenMM (engine)
conda install -c conda-forge openmm

# OpenFF toolkit + Sage FF
pip install openff-toolkit
pip install openmmforcefields  # bridges OpenFF → OpenMM + GAFF2

# AMBER Tools (antechamber / tleap)
conda install -c conda-forge ambertools

# acpype (antechamber wrapper → GROMACS/AMBER topology)
conda install -c conda-forge acpype

# ParmEd (topology manipulation)
pip install parmed

# Verify
python -c "import openmm; print(openmm.__version__)"
python -c "from openff.toolkit import Molecule; print('OpenFF OK')"

Related Skills

• ase — geometry optimization with QM calculators (ORCA, xTB, GPAW)
• mdanalysis — trajectory analysis after MD runs
• docking — pre-docking protein prep; post-MD ensemble docking
• scientific-skills:pymatgen — periodic materials, materials force fields (ReaxFF)
• scripts: chem_3d.py — RDKit 3D conformer generation (pre-MD structure)