md-force-fields

MD Force Fields

Description

This skill covers selection, validation, and deployment of interatomic force fields for molecular dynamics: the physical basis and transferability limits of every major functional form (EAM, MEAM, Tersoff, Stillinger-Weber, Buckingham, COMB, ReaxFF, AMBER, CHARMM, OPLS, GROMOS, and ML potentials); parameter-file sourcing and provenance verification; unit and format compatibility across LAMMPS, GROMACS, OpenMM, and ASE; mixing rules, long-range electrostatics, cutoff conventions, neighbor-list settings, and bonded constraints; and validation of any force field against DFT, experiment, or higher-level simulation. Invoke this skill when selecting a force field for a new system, diagnosing a simulation failure that may originate from a wrong or misapplied force field, or verifying the transferability of an existing parameterization to a new application domain.

Domain Context

A force field is a function that maps a set of atomic positions {rᵢ} to a potential energy U({rᵢ}) and, by differentiation, to forces Fᵢ = -∂U/∂rᵢ. Every force field encodes assumptions about which physical interactions are important and which are negligible. These assumptions are baked into the functional form; they cannot be corrected by changing parameter values alone.

The central issue with force fields is transferability: a parameterization fitted to reproduce a set of reference properties in one regime is not guaranteed to be accurate in a different regime. The reference data (crystal structures, elastic constants, bond lengths, heats of formation, vibrational frequencies) used to fit the parameters determines the validity domain. Using the force field outside that domain is extrapolation — the errors are unbounded and often silent.

Hierarchy of approximations, from most to least severe:

1. Born-Oppenheimer surface: All classical force fields assume the ground-state PES. Electronically excited states, non-adiabatic dynamics, metallic-band effects on bonding, and charge-transfer-driven structural changes are outside the Born-Oppenheimer classical picture entirely.
2. Fixed topology vs. reactive: Fixed-topology force fields (AMBER, CHARMM, EAM, Tersoff) assign bonds at the start of the simulation and never break or form them. Reactive force fields (ReaxFF, COMB) allow bond-order to vary continuously. ML potentials can be either reactive or non-reactive depending on the training data.
3. Many-body vs. pairwise: Pure pairwise potentials (LJ, Buckingham) cannot reproduce the Cauchy pressure discrepancy in metals (C₁₂ ≠ C₄₄) or the angular dependence of covalent bonds. Many-body terms (EAM embedding energy, Tersoff bond-order, AMBER dihedral) correct specific deficiencies but add parameters.
4. Classical nuclei: All MD force fields integrate classical equations of motion. Zero-point energy and nuclear tunneling are neglected. This fails for light atoms (H, He, Li) at low temperature.
5. Mean-field charges: Classical force fields use fixed partial charges (AMBER, CHARMM) or fluctuating charges (ReaxFF QEq, COMB). Neither is a first-principles electron density; both fail when charge delocalization or metallic screening becomes important.