forked from lijiext/lammps
87 lines
3.2 KiB
Plaintext
87 lines
3.2 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Commands_all.html)
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:line
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fix spring/chunk command :h3
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[Syntax:]
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fix ID group-ID spring/chunk K chunkID comID :pre
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ID, group-ID are documented in "fix"_fix.html command
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spring/chunk = style name of this fix command
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K = spring constant for each chunk (force/distance units)
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chunkID = ID of "compute chunk/atom"_compute_chunk_atom.html command
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comID = ID of "compute com/chunk"_compute_com_chunk.html command :ul
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[Examples:]
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fix restrain all spring/chunk 100 chunkID comID :pre
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[Description:]
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Apply a spring force to the center-of-mass (COM) of chunks of atoms as
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defined by the "compute chunk/atom"_compute_chunk_atom.html command.
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Chunks can be molecules or spatial bins or other groupings of atoms.
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This is a way of tethering each chunk to its initial COM coordinates.
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The {chunkID} is the ID of a compute chunk/atom command defined in the
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input script. It is used to define the chunks. The {comID} is the ID
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of a compute com/chunk command defined in the input script. It is
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used to compute the COMs of each chunk.
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At the beginning of the first "run"_run.html or
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"minimize"_minimize.html command after this fix is defined, the
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initial COM of each chunk is calculated and stored as R0m, where M is
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the chunk number. Thereafter, at every timestep (or minimization
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iteration), the current COM of each chunk is calculated as Rm. A
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restoring force of magnitude K (Rm - R0m) Mi / Mm is applied to each
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atom in each chunk where {K} is the specified spring constant, Mi is
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the mass of the atom, and Mm is the total mass of all atoms in the
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chunk. Note that {K} thus represents the spring constant for the
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total force on each chunk of atoms, not for a spring applied to each
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atom.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {energy} option is supported by this
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fix to add the energy stored in all the springs to the system's potential
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energy as part of "thermodynamic output"_thermo_style.html.
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The "fix_modify"_fix_modify.html {respa} option is supported by this
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fix. This allows to set at which level of the "r-RESPA"_run_style.html
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integrator the fix is adding its forces. Default is the outermost level.
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This fix computes a global scalar which can be accessed by various
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"output commands"_Howto_output.html. The scalar is the energy of all
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the springs, i.e. 0.5 * K * r^2 per-spring.
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The scalar value calculated by this fix is "extensive".
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No parameter of this fix can be used with the {start/stop} keywords of
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the "run"_run.html command.
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The forces due to this fix are imposed during an energy minimization,
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invoked by the "minimize"_minimize.html command.
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NOTE: If you want the spring energies to be included in the total
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potential energy of the system (the quantity being minimized), you
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MUST enable the "fix_modify"_fix_modify.html {energy} option for this
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fix.
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[Restrictions:] none
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[Related commands:]
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"fix spring"_fix_spring.html, "fix spring/self"_fix_spring_self.html,
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"fix spring/rg"_fix_spring_rg.html
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[Default:] none
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