Syntax:
fix ID group spring x y z k
Examples:
fix pull ligand spring 0.0 1.0 1.0 5.0
Description:
Apply a spring force to a group of atoms. An example is an umbrella force on an small molecule. A second example is lightly tethering a large group of atoms (e.g. all the solvent or a large molecule) to the center of the simulation box so that it doesn't wander away over the course of a long simulation.
Each timestep, the center of mass R of the group of atoms is computed, taking account of wrap-around in a periodic simulation box. A restoring force of magnitude k (R - R0) Mi / M is applied to each atom in the group where k is the specified force constant, R0 is the tethering point of the spring specified by (x,y,z), Mi is the mass of the atom, and M is the total mass of all atoms in the group. Note that k thus represents the total force on the group of atoms, not a per-atom force.
Any of the x,y,z values can be specified as NULL which means do not include that dimension in the distance calculation or force application.
Restrictions: none
Related commands:
Default: none