fix dt/reset command

Syntax

fix ID group-ID dt/reset N Tmin Tmax Xmax keyword values ...

• ID, group-ID are documented in fix command

• dt/reset = style name of this fix command

• N = re-compute dt every N timesteps

• Tmin = minimum dt allowed which can be NULL (time units)

• Tmax = maximum dt allowed which can be NULL (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 = emax or units

emax value = Emax
  Emax = maximum kinetic energy change for an atom in one timestep (energy units)
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
fix 5 all dt/reset 5 NULL 0.001 0.5 emax 30 units box

Description

Reset the timestep size every N steps during a run, so that no atom moves further than the specified Xmax distance, based on current atom velocities and forces. Optionally an additional criterion is imposed by the emax keyword, so that no atom’s kinetic energy changes by more than the specified Emax.

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 manner.

For each atom, the timestep is computed that would cause it to displace Xmax on the next integration step, as a function of its current velocity and force. Since performing this calculation exactly would require the solution to a quartic equation, a cheaper estimate is generated. The estimate is conservative in that the atom’s displacement is guaranteed not to exceed Xmax, though it may be smaller.

In addition if the emax keyword is used, the specified Emax value is enforced as a limit on how much an atom’s kinetic energy can change. If the timestep required is even smaller than for the Xmax displacement, then the smaller timestep is used.

Given this putative timestep for each atom, the minimum timestep value across all atoms is computed. Then the Tmin and Tmax bounds are applied, if specified. If one (or both) is specified as NULL, it is not applied.

When the run style is respa, this fix resets the outer loop (largest) timestep, which is the same timestep that the timestep command sets.

Note that the cumulative simulation time (in time units), which accounts for changes in the timestep size as a simulation proceeds, can be accessed by the thermo_style time keyword.

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.

This fix computes a global scalar which can be accessed by various output commands. The scalar stores the last timestep on which the timestep was reset to a new value.

The scalar value calculated by this fix is “intensive”.

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

none

Related commands

timestep

Default

The option defaults are units = lattice, and no emax kinetic energy limit.