forked from lijiext/lammps
84 lines
3.0 KiB
Plaintext
84 lines
3.0 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/self command :h3
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[Syntax:]
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fix ID group-ID spring/self K dir :pre
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ID, group-ID are documented in "fix"_fix.html command
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spring/self = style name of this fix command
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K = spring constant (force/distance units)
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dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz) :ul
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[Examples:]
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fix tether boundary-atoms spring/self 10.0
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fix zrest move spring/self 10.0 z :pre
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[Description:]
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Apply a spring force independently to each atom in the group to tether
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it to its initial position. The initial position for each atom is its
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location at the time the fix command was issued. At each timestep,
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the magnitude of the force on each atom is -Kr, where r is the
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displacement of the atom from its current position to its initial
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position. The distance r correctly takes into account any crossings
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of periodic boundary by the atom since it was in its initial
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position.
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With the (optional) dir flag, one can select in which direction the
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spring force is applied. By default, the restraint is applied in all
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directions, but it can be limited to the xy-, xz-, yz-plane and the
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x-, y-, or z-direction, thus restraining the atoms to a line or a
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plane, respectively.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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This fix writes the original coordinates of tethered atoms to "binary
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restart files"_restart.html, so that the spring effect will be the
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same in a restarted simulation. See the
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"read_restart"_read_restart.html command for info on how to re-specify
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a fix in an input script that reads a restart file, so that the
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operation of the fix continues in an uninterrupted fashion.
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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 the per-atom springs to the system's
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potential energy as part of "thermodynamic output"_thermo_style.html.
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The "fix_modify"_fix_modify.html {respa} option is supported by
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this 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 an energy which is
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the sum of the spring energy for each atom, where the per-atom energy
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is 0.5 * K * r^2. The scalar value calculated by this fix is
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"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 per-atom spring energy to be included in the
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total potential energy of the system (the quantity being minimized),
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you MUST enable the "fix_modify"_fix_modify.html {energy} option for
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this fix.
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[Restrictions:] none
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[Related commands:]
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"fix drag"_fix_drag.html, "fix spring"_fix_spring.html,
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"fix smd"_fix_smd.html, "fix spring/rg"_fix_spring_rg.html
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[Default:] none
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