The boun record defines sets of distance constraints which can be applied during molecular dynamics simulations. It is not necessary to have this record in order to run constrained molecular dynamics. It is possible to use the equilibrium bond lengths in the bond record to constrain bond distances for instance. The boun record has the form

boun	Nclass
	1	Nbound
		atomi	atomj	dij
		...	...	...
	...	...

where N_class is the number of classes of constraints. Currently only one class, distance constraints, is available. Thus, N_class must have the value 1. Following the header line is the data for the first class (distance constraints). The data starts with the class identifier which is the value 1, followed by the number of distance bounds N_bound, then N_bound lines containing three fields each. The fields are the atom pairs to be bounded atom_i and atom_j, and the distance to which they are to be bounded d_ij.

boun   1

1   5

1   2   34.0
2   3   34.0
3   4   34.0
4   5   34.0
5   1   34.0

Five atoms are laid single file on a closed curve (of indeterminate shape) and each atom is constrained to be 34Å from its two neighbors.