forked from lijiext/lammps
93 lines
3.3 KiB
Plaintext
93 lines
3.3 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,Section_commands.html#comm)
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:line
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fix dt/reset command :h3
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[Syntax:]
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fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ... :pre
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ID, group-ID are documented in "fix"_fix.html command
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dt/reset = style name of this fix command
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N = recompute dt every N timesteps
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Tmin = minimum dt allowed (can be NULL) (time units)
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Tmax = maximum dt allowed (can be NULL) (time units)
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Xmax = maximum distance for an atom to move in one timestep (distance units)
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zero or more keyword/value pairs may be appended
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keyword = {units} :ul
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{units} value = {lattice} or {box}
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lattice = Xmax is defined in lattice units
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box = Xmax is defined in simulation box units :pre
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[Examples:]
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fix 5 all dt/reset 10 1.0e-5 0.01 0.1
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fix 5 all dt/reset 10 0.01 2.0 0.2 units box :pre
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[Description:]
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Reset the timestep size every N steps during a run, so that no atom
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moves further than Xmax, based on current atom velocities and forces.
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This can be useful when starting from a configuration with overlapping
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atoms, where forces will be large. Or it can be useful when running
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an impact simulation where one or more high-energy atoms collide with
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a solid, causing a damage cascade.
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This fix overrides the timestep size setting made by the
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"timestep"_timestep.html command. The new timestep size {dt} is
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computed in the following manner.
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For each atom, the timestep is computed that would cause it to
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displace {Xmax} on the next integration step, as a function of its
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current velocity and force. Since performing this calculation exactly
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would require the solution to a quartic equation, a cheaper estimate
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is generated. The estimate is conservative in that the atom's
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displacement is guaranteed not to exceed {Xmax}, though it may be
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smaller.
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Given this putative timestep for each atom, the minimum timestep value
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across all atoms is computed. Then the {Tmin} and {Tmax} bounds are
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applied, if specified. If one (or both) is specified as NULL, it is
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not applied.
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When the "run style"_run_style.html is {respa}, this fix resets the
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outer loop (largest) timestep, which is the same timestep that the
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"timestep"_timestep.html command sets.
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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. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix.
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The current timestep size is stored as a scalar quantity by this fix.
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The cumulative simulation time (in time units) is stored as the first
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element of a vector. Both these quantities can be accessed by various
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"output commands"_Section_howto.html#4_15. The scalar and vector
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values calculated by this fix are "intensive", meaning they are
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independent of the number of atoms in the simulation.
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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. This fix is not invoked during "energy
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minimization"_minimize.html.
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[Restrictions:]
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The cumulative time is zeroed when the fix is created and
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continuously accrues thereafter. Using the
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"reset_timestep"_reset_timestep.html command while this fix is defined
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will mess up the time accumulation.
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[Related commands:]
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"timestep"_timestep.html
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[Default:]
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The option defaults is units = lattice.
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