constraint -define a constraint on one or more bond lengths

SYNOPSIS

  constraint distance atom1 atom2 distance [distance_derivative]
  constraint multidistance weight atom1 atom2
      [weight atom1 atom2] distance [distance_derivative]

DESCRIPTION

The constraint command defines a bond length constraint and adds it to the list of constraints. There are currently two types of constraints available.

Distance contraint
The distance between atoms a1 and a2 can be fixed to d using the syntax

    constraint distance a1 a2 d

A rate of increase (or decrease) of the distance can optionally be specified as in

    constraint distance a1 a2 d v

In this case, the quantity v*dt will be added to d after each MD step. The distance d and the rate of increase v must be specified in atomic units.

Multi-distance contraint
The weighted sum of distances between pairs of atoms (a_i, b_i), i=1,..,N can be fixed to d using the syntax

    constraint multidistance w_1 a_1 b_1 w_2 a_2 b_2 .... w_N a_N b_N d

In this case, the following equality will be enforced

    sum_i w_i * distance(a_i, b_i) = d

NOTE

The syntax of the constraint command has changed as of version 1.11.0. The constraint type ("distance" or "multidistance") must now be specified after the "constraint" keyword.

RELATED INFORMATION

list_constraints, del_constraint, dt