Force fields

The force field is the physics contract.

It defines how atoms are typed, how bonds and angles resist deformation, how charges interact, and how nonbonded atoms attract or repel each other. PyMACS highlights the CHARMM family, especially CHARMM36 and CHARMM36/LJ-PME, with CGenFF for small molecules.

PyMACS primary family

CHARMM36

A widely used additive force-field family for proteins, nucleic acids, lipids, carbohydrates, ions, and related biomolecular systems.

Strengths

  • broad biomolecular coverage
  • well established in GROMACS workflows
  • strong pairing with CGenFF for ligands

Watch for

Users still need compatible residue names, sensible protonation choices, and ligand parameters that match the chemistry being simulated.

Highlighted PyMACS variant

CHARMM36/LJ-PME

A CHARMM36-compatible variant that treats long-range Lennard-Jones interactions with particle-mesh Ewald methods for systems parameterized for that approach.

Strengths

  • consistent long-range dispersion treatment
  • efficient nonbonded settings
  • bundled in the PyMACS repository

Watch for

Use it when the force-field files and system class are compatible; do not mix assumptions across variants casually.

Small-molecule parameterization

CGenFF

The CHARMM General Force Field assigns atom types, charges, bonded terms, and penalty scores for drug-like molecules and cofactors.

Strengths

  • good coverage of medicinal chemistry scaffolds
  • penalty scores flag uncertain parameters
  • fits CHARMM-family workflows

Watch for

High penalty scores may require expert refinement before results are publication-grade.

Common alternative

AMBER

A major force-field family often used for proteins, nucleic acids, and biomolecular simulations, with GAFF commonly used for small molecules.

Strengths

  • large user community
  • strong protein and nucleic-acid history
  • many tutorials and tools

Watch for

AMBER-style parameterization choices are not interchangeable with CHARMM choices without careful conversion and validation.

Common alternative

OPLS

A family used in biomolecular and small-molecule simulations, often associated with liquid-phase and organic chemistry parameterization traditions.

Strengths

  • useful small-molecule coverage
  • established in several MD packages
  • clear bonded and nonbonded forms

Watch for

Topology generation and ligand workflows differ from CHARMM/CGenFF conventions.

Common alternative

GROMOS

A force-field family with a long history in biomolecular simulation and GROMACS-oriented workflows.

Strengths

  • historically important in GROMACS
  • efficient united-atom options
  • well known in European MD communities

Watch for

United-atom and all-atom assumptions can affect how results compare with CHARMM or AMBER simulations.

Coarse-grained alternative

Martini

A coarse-grained force field that groups atoms into beads, allowing larger systems or longer timescales than typical all-atom MD.

Strengths

  • longer timescale sampling
  • large membrane and assembly studies
  • reduced computational cost

Watch for

Coarse-grained simulations answer different questions than all-atom PyMACS/CHARMM workflows.

Why PyMACS uses CHARMM

Consistency from protein to ligand.

The PyMACS workflow is built around CHARMM36-family biomolecular topology generation and CGenFF-compatible ligand parameterization. That keeps protein, nucleic-acid, cofactor, and drug-like molecule handling inside one conceptual family.

Beginner rule: choose one force-field family for the whole system and keep every topology, ligand parameter, water model, ion model, and MDP assumption compatible with that choice.