Some force fields define additional restrictions concerning which particles should interact together and which ones should not. For example, sometimes bonded particles should not interact through electrostatic potential, or some interactions should only be taken in account for particles not in the same molecule. The way to specify this is to use restrictions. Restrictions can be used in two places: in the [pairs] section, and in the [coulomb] section. In both cases, they are specified with the restriction keyword, and one of the possible values.

O-O = {type = "lj", sigma = "3 A", epsilon = "123 kJ/mol", restriction = {scale14 = 0.5}}

ewald = {cutoff = "8 A", kmax = 6}
restriction = "intermolecular"

The possible values for restriction are:

  • "intramolecular" or "intra-molecular" to act only on particles that are in the same molecule;
  • "intermolecular" or "inter-molecular" to act only on particles that are NOT in the same molecule;
  • "exclude12" to exclude particles directly bonded together;
  • "exclude13" to exclude particles directly bonded together or forming an angle;
  • "exclude14" to exclude particles directly bonded together; forming an angle or a dihedral angle;
  • {scale14 = <scaling>} works like exclude13, i.e. intramolecular interactions between three neighboring particles (connected by two bonds) will not be computed. Additionally, interactions between the first and the forth (hence the 14 in scale14) particle will be computed, but using scaled energies and forces. This simply means that the energies and forces are multiplied (linear scaling) by the given scaling factor, which must be between 0 and 1.