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Restraints interface

This interface serves as a modifier for other calculations. It allows to add harmonic restraints in internal coordinates. The restraints can be applied not only to single atoms, but also to groups of atoms, acting on centers of mass of the groups.

Input for this interface uses only one keyword, restraints, that contains the list of restraints as an array. Each entry is a semicolo-separated list:

distance|angle|torsion|distance_difference; value; force_constant; atom_selection1; atom_selection2; ...

First is specification of the type of internal coordinate, then its value in energy minimum (distance in Angstroms, angles in degrees), force constant (kcal/angstrom, kcal/radian) and then selections of the atoms or groups.

The distance_difference coordinate is calculated as a difference between two interatomic distances: x = distance(1,2) - distance (3,4). Therefore, it requires four atom selections.



In this example, we use the restraints interface itself (not as modifier) to define simple forcefield for a hydrogen peroxide molecule. To define the optimum geometry, we use all three types of restraints available.

An very bad guess of the geometry


H  1.0 0.0 0.0
O  0.0 1.0 0.0
O  0.0 0.0 5.0
H  1.0 0.0 8.0

can be optimized using the following input file:

interface: restraints
  - distance; 0.95; 10; 1; 2
  - distance; 1.47; 10; 2; 3
  - distance; 0.95; 10; 3; 4
  - angle; 94.8; 10; 1; 2; 3
  - angle; 94.8; 10; 2; 3; 4
  - torsion; 111.5; 10; 1;2;3;4