Syntax: +
+fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ... ++
units value = lattice or box + lattice = Xmax is defined in lattice units + box = Xmax is defined in simulation box units ++
Examples: +
+fix 5 all dt/reset 10 1.0e-5 0.01 0.1 +fix 5 all dt/reset 10 0.01 2.0 0.2 units box ++
Description: +
+Reset the timestep size every N steps during a run, based on current +atom velocities and forces. This can be useful when starting from a +configuration with overlapping atoms, where forces will be large. Or +it can be useful when running an impact simulation where one or more +high-energy atoms collide with a solid, causing a damage cascade. +
+This fix overrides the timestep size setting made by the +timestep command. The new timestep size dt is +computed in the following way. +
+Vmax is the maximum velocity; Amax is the maximum acceleration = +force/mass. Note that Tmin or Tmax can be specified as INF, in which +case one or both of the last 2 checks will not be performed. +
+When the run style is respa, this fix resets the +outer loop (largest) timestep, which is the same timestep that the +timestep command sets. +
+Restart, fix_modify, output, run start/stop, minimize info: +
+No information about this fix is written to binary restart +files. None of the fix_modify options +are relevant to this fix. +
+The current timestep size is stored as a scalar quantity by this fix. +The cummulative simulation time (in time units) is stored as the first +element of a vector. Both these quantities can be accessed by various +output commands. +
+No parameter of this fix can be used with the start/stop keywords of +the run command. This fix is not invoked during energy +minimization. +
+Restrictions: +
+The cummulative time is zeroed when the fix is created and +continuously accrues thereafter. Using the +reset_timestep command while this fix is defined +will mess up the time accumulation. +
+Related commands: +
+timestep +
+Default: +
+The option defaults is units = lattice. +
+ diff --git a/doc/fix_dt_reset.txt b/doc/fix_dt_reset.txt new file mode 100644 index 0000000000..9270e19c6c --- /dev/null +++ b/doc/fix_dt_reset.txt @@ -0,0 +1,88 @@ +"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c + +:link(lws,http://lammps.sandia.gov) +:link(ld,Manual.html) +:link(lc,Section_commands.html#comm) + +:line + +fix dt/reset command :h3 + +[Syntax:] + +fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ... :pre + +ID, group-ID are documented in "fix"_fix.html command +dt/reset = style name of this fix command +N = recompute dt every N timesteps +Tmin = minimum dt allowed (can be INF) (time units) +Tmax = maximum dt allowed (can be INF) (time units) +Xmax = maximum distance for an atom to move in one timestep (distance units) +zero or more keyword/value pairs may be appended +keyword = {units} :ul + {units} value = {lattice} or {box} + lattice = Xmax is defined in lattice units + box = Xmax is defined in simulation box units :pre + +[Examples:] + +fix 5 all dt/reset 10 1.0e-5 0.01 0.1 +fix 5 all dt/reset 10 0.01 2.0 0.2 units box :pre + +[Description:] + +Reset the timestep size every N steps during a run, based on current +atom velocities and forces. This can be useful when starting from a +configuration with overlapping atoms, where forces will be large. Or +it can be useful when running an impact simulation where one or more +high-energy atoms collide with a solid, causing a damage cascade. + +This fix overrides the timestep size setting made by the +"timestep"_timestep.html command. The new timestep size {dt} is +computed in the following way. + +compute Vmax of any atom in group +compute Amax of any atom in group +dt1 = Xmax/Vmax +dt2 = sqrt(2 Xmax/Amax) +new dt = MIN(dt1,dt2) +if dt < Tmin, dt = Tmin +if dt > Tmax, dt = Tmax :ul + +Vmax is the maximum velocity; Amax is the maximum acceleration = +force/mass. Note that Tmin or Tmax can be specified as INF, in which +case one or both of the last 2 checks will not be performed. + +When the "run style"_run_style.html is {respa}, this fix resets the +outer loop (largest) timestep, which is the same timestep that the +"timestep"_timestep.html command sets. + +[Restart, fix_modify, output, run start/stop, minimize info:] + +No information about this fix is written to "binary restart +files"_restart.html. None of the "fix_modify"_fix_modify.html options +are relevant to this fix. + +The current timestep size is stored as a scalar quantity by this fix. +The cummulative simulation time (in time units) is stored as the first +element of a vector. Both these quantities can be accessed by various +"output commands"_Section_howto.html#4_15. + +No parameter of this fix can be used with the {start/stop} keywords of +the "run"_run.html command. This fix is not invoked during "energy +minimization"_minimize.html. + +[Restrictions:] + +The cummulative time is zeroed when the fix is created and +continuously accrues thereafter. Using the +"reset_timestep"_reset_timestep.html command while this fix is defined +will mess up the time accumulation. + +[Related commands:] + +"timestep"_timestep.html + +[Default:] + +The option defaults is units = lattice.