# Interaction function and force fields¶

To accommodate the potential functions used in some popular force fields (see Interaction function and force fields), GROMACS offers a choice of functions, both for non-bonded interaction and for dihedral interactions. They are described in the appropriate subsections.

The potential functions can be subdivided into three parts

*Non-bonded*: Lennard-Jones or Buckingham, and Coulomb or modified Coulomb. The non-bonded interactions are computed on the basis of a neighbor list (a list of non-bonded atoms within a certain radius), in which exclusions are already removed.*Bonded*: covalent bond-stretching, angle-bending, improper dihedrals, and proper dihedrals. These are computed on the basis of fixed lists.*Restraints*: position restraints, angle restraints, distance restraints, orientation restraints and dihedral restraints, all based on fixed lists.*Applied Forces*: externally applied forces, see chapter Special Topics.

- Non-bonded interactions
- Bonded interactions
- Bond stretching
- Morse potential bond stretching
- Cubic bond stretching potential
- FENE bond stretching potential
- Harmonic angle potential
- Cosine based angle potential
- Restricted bending potential
- Urey-Bradley potential
- Linear Angle potential
- Bond-Bond cross term
- Bond-Angle cross term
- Quartic angle potential
- Improper dihedrals
- Proper dihedrals
- Tabulated bonded interaction functions

- Restraints
- Polarization
- Free energy interactions
- Methods
- Virtual interaction sites
- On top of an atom
- As a linear combination of two atoms (
`Fig. %s`

2) - On the line through two atoms, with a fixed distance (
`Fig. %s`

2fd) - As a linear combination of three atoms (
`Fig. %s`

3) - In the plane of three atoms, with a fixed distance (
`Fig. %s`

3fd) - In the plane of three atoms, with a fixed angle and distance (
`Fig. %s`

3fad) - As a non-linear combination of three atoms, out of plane (
`Fig. %s`

3out) - From four atoms, with a fixed distance, see separate
`Fig. %s`

- A linear combination of \(N\) atoms with relative weights \(a_i\)

- Long Range Electrostatics
- Long Range Van der Waals interactions
- Force field