git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@1004 f3b2605a-c512-4ea7-a41b-209d697bcdaa
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sjplimp
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doc/fix_dt_reset.html
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doc/fix_dt_reset.html
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<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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</CENTER>
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<HR>
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<H3>fix dt/reset command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>dt/reset = style name of this fix command
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<LI>N = recompute dt every N timesteps
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<LI>Tmin = minimum dt allowed (can be INF) (time units)
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<LI>Tmax = maximum dt allowed (can be INF) (time units)
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<LI>Xmax = maximum distance for an atom to move in one timestep (distance units)
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>units</I>
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</UL>
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<PRE> <I>units</I> value = <I>lattice</I> or <I>box</I>
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lattice = Xmax is defined in lattice units
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box = Xmax is defined in simulation box units
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>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
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Reset the timestep size every N steps during a run, based on current
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atom velocities and forces. This can be useful when starting from a
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configuration with overlapping atoms, where forces will be large. Or
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it can be useful when running an impact simulation where one or more
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high-energy atoms collide with a solid, causing a damage cascade.
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</P>
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<P>This fix overrides the timestep size setting made by the
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<A HREF = "timestep.html">timestep</A> command. The new timestep size <I>dt</I> is
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computed in the following way.
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</P>
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<UL><LI>compute Vmax of any atom in group
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<LI>compute Amax of any atom in group
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<LI>dt1 = Xmax/Vmax
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<LI>dt2 = sqrt(2 Xmax/Amax)
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<LI>new dt = MIN(dt1,dt2)
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<LI>if dt < Tmin, dt = Tmin
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<LI>if dt > Tmax, dt = Tmax
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</UL>
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<P>Vmax is the maximum velocity; Amax is the maximum acceleration =
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force/mass. Note that Tmin or Tmax can be specified as INF, in which
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case one or both of the last 2 checks will not be performed.
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</P>
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<P>When the <A HREF = "run_style.html">run style</A> is <I>respa</I>, this fix resets the
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outer loop (largest) timestep, which is the same timestep that the
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<A HREF = "timestep.html">timestep</A> command sets.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix.
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</P>
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<P>The current timestep size is stored as a scalar quantity by this fix.
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The cummulative 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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<A HREF = "Section_howto.html#4_15">output commands</A>.
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>The cummulative time is zeroed when the fix is created and
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continuously accrues thereafter. Using the
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<A HREF = "reset_timestep.html">reset_timestep</A> command while this fix is defined
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will mess up the time accumulation.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "timestep.html">timestep</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults is units = lattice.
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</P>
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</HTML>
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88
doc/fix_dt_reset.txt
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88
doc/fix_dt_reset.txt
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"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 INF) (time units)
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Tmax = maximum dt allowed (can be INF) (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, based on current
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atom velocities and forces. This can be useful when starting from a
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configuration with overlapping atoms, where forces will be large. Or
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it can be useful when running an impact simulation where one or more
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high-energy atoms collide with 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 way.
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compute Vmax of any atom in group
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compute Amax of any atom in group
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dt1 = Xmax/Vmax
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dt2 = sqrt(2 Xmax/Amax)
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new dt = MIN(dt1,dt2)
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if dt < Tmin, dt = Tmin
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if dt > Tmax, dt = Tmax :ul
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Vmax is the maximum velocity; Amax is the maximum acceleration =
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force/mass. Note that Tmin or Tmax can be specified as INF, in which
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case one or both of the last 2 checks will not be performed.
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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 cummulative 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.
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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 cummulative 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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