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fix spring/chunk command :h3

[Syntax:]

fix ID group-ID spring/chunk K chunkID comID :pre

ID, group-ID are documented in "fix"_fix.html command
spring/chunk = style name of this fix command
K = spring constant for each chunk (force/distance units)
chunkID = ID of "compute chunk/atom"_compute_chunk_atom.html command
comID = ID of "compute com/chunk"_compute_com_chunk.html command :ul

[Examples:]

fix restrain all spring/chunk 100 chunkID comID :pre

[Description:]

Apply a spring force to the center-of-mass (COM) of chunks of atoms as
defined by the "compute chunk/atom"_compute_chunk_atom.html command.
Chunks can be molecules or spatial bins or other groupings of atoms.
This is a way of tethering each chunk to its initial COM coordinates.

The {chunkID} is the ID of a compute chunk/atom command defined in the
input script.  It is used to define the chunks.  The {comID} is the ID
of a compute com/chunk command defined in the input script.  It is
used to compute the COMs of each chunk.

At the beginning of the first "run"_run.html or
"minimize"_minimize.html command after this fix is defined, the
initial COM of each chunk is calculated and stored as R0m, where M is
the chunk number.  Thereafter, at every timestep (or minimization
iteration), the current COM of each chunk is calculated as Rm.  A
restoring force of magnitude K (Rm - R0m) Mi / Mm is applied to each
atom in each chunk where {K} is the specified spring constant, Mi is
the mass of the atom, and Mm is the total mass of all atoms in the
chunk.  Note that {K} thus represents the spring constant for the
total force on each chunk of atoms, not for a spring applied to each
atom.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.

The "fix_modify"_fix_modify.html {energy} option is supported by this
fix to add the energy stored in all the springs to the system's potential
energy as part of "thermodynamic output"_thermo_style.html.

The "fix_modify"_fix_modify.html {respa} option is supported by this
fix. This allows to set at which level of the "r-RESPA"_run_style.html
integrator the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar which can be accessed by various
"output commands"_Howto_output.html.  The scalar is the energy of all
the springs, i.e. 0.5 * K * r^2 per-spring.

The scalar value calculated by this fix is "extensive".

No parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.

The forces due to this fix are imposed during an energy minimization,
invoked by the "minimize"_minimize.html command.

NOTE: If you want the spring energies to be included in the total
potential energy of the system (the quantity being minimized), you
MUST enable the "fix_modify"_fix_modify.html {energy} option for this
fix.

[Restrictions:] none

[Related commands:]

"fix spring"_fix_spring.html, "fix spring/self"_fix_spring_self.html,
"fix spring/rg"_fix_spring_rg.html

[Default:] none
